Minor formatting updates for gvisor.dev.

* Aggregate architecture Overview in "What is gVisor?" as it makes more sense
  in one place.

* Drop "user-space kernel" and use "application kernel". The term "user-space
  kernel" is confusing when some platform implementation do not run in
  user-space (instead running in guest ring zero).

* Clear up the relationship between the Platform page in the user guide and the
  Platform page in the architecture guide, and ensure they are cross-linked.

* Restore the call-to-action quick start link in the main page, and drop the
  GitHub link (which also appears in the top-right).

* Improve image formatting by centering all doc and blog images, and move the
  image captions to the alt text.

PiperOrigin-RevId: 311845158

7 files changed, 25 insertions, 38 deletions

diff --git a/website/BUILD b/website/BUILD
index d6afd5f44..c97b2560b 100644
--- a/website/BUILD
+++ b/website/BUILD
@@ -138,7 +138,6 @@ docs(
         "//g3doc:community",
         "//g3doc:index",
         "//g3doc:roadmap",
-        "//g3doc/architecture_guide:index",
         "//g3doc/architecture_guide:performance",
         "//g3doc/architecture_guide:platforms",
         "//g3doc/architecture_guide:resources",
diff --git a/website/_layouts/docs.html b/website/_layouts/docs.html
index 33ea8e1de..549305089 100644
--- a/website/_layouts/docs.html
+++ b/website/_layouts/docs.html
@@ -51,7 +51,9 @@ categories:
         <a href="https://github.com/google/gvisor/issues/new?title={{page.title | url_encode}}" target="_blank"><i class="fab fa-github fa-fw"></i> Create issue</a>
         </p>
       {% endif %}
+      <div class="docs-content">
       {{ content }}
+      </div>
     </div>
   </div>
 </div>
diff --git a/website/_sass/front.scss b/website/_sass/front.scss
index 44a7e3473..0e4208f3c 100644
--- a/website/_sass/front.scss
+++ b/website/_sass/front.scss
@@ -4,12 +4,14 @@
   background-repeat: no-repeat;
   background-size: cover;
   background-blend-mode: darken;
-  background-color: rgba(0, 0, 0, 0.1);
+  background-color: rgba(0, 0, 0, 0.3);
 
   p {
     color: #fff;
     margin-top: 0;
     margin-bottom: 0;
     font-weight: 300;
+    font-size: 24px;
+    line-height: 30px;
   }
 }
diff --git a/website/_sass/style.scss b/website/_sass/style.scss
index 520ea469a..4deb945d4 100644
--- a/website/_sass/style.scss
+++ b/website/_sass/style.scss
@@ -142,3 +142,13 @@ table th {
   margin-top: 10px;
   margin-bottom: 20px;
 }
+
+.docs-content * img {
+  display: block;
+  margin: 20px auto;
+}
+
+.blog-content * img {
+  display: block;
+  margin: 20px auto;
+}
diff --git a/website/blog/2019-11-18-security-basics.md b/website/blog/2019-11-18-security-basics.md
index ed6d97ffe..fbdd511dd 100644
--- a/website/blog/2019-11-18-security-basics.md
+++ b/website/blog/2019-11-18-security-basics.md
@@ -56,15 +56,9 @@ in combination: redundant walls, scattered draw bridges, small bottle-neck
 entrances, moats, etc.
 
 A simplified version of the design is below
-([more detailed version](/docs/architecture_guide/))[^2]:
+([more detailed version](/docs/))[^2]:
 
---------------------------------------------------------------------------------
-
-![Figure 1](/assets/images/2019-11-18-security-basics-figure1.png)
-
-Figure 1: Simplified design of gVisor.
-
---------------------------------------------------------------------------------
+![Figure 1](/assets/images/2019-11-18-security-basics-figure1.png "Simplified design of gVisor.")
 
 In order to discuss design principles, the following components are important to
 know:
@@ -134,13 +128,7 @@ minimum level of permission is required for it to perform its function.
 Specifically, the closer you are to the untrusted application, the less
 privilege you have.
 
---------------------------------------------------------------------------------
-
-![Figure 2](/assets/images/2019-11-18-security-basics-figure2.png)
-
-Figure 2: runsc components and their privileges.
-
---------------------------------------------------------------------------------
+![Figure 2](/assets/images/2019-11-18-security-basics-figure2.png "runsc components and their privileges.")
 
 This is evident in how runsc (the drop in gVisor binary for Docker/Kubernetes)
 constructs the sandbox. The Sentry has the least privilege possible (it can't
@@ -222,15 +210,7 @@ the host Linux syscalls. In other words, with gVisor, applications get the vast
 majority (and growing) functionality of Linux containers for only 68 possible
 syscalls to the Host OS. 350 syscalls to 68 is attack surface reduction.
 
---------------------------------------------------------------------------------
-
-![Figure 3](/assets/images/2019-11-18-security-basics-figure3.png)
-
-Figure 3: Reduction of Attack Surface of the Syscall Table. Note that the
-Senty's Syscall Emulation Layer keeps the Containerized Process from ever
-calling the Host OS.
-
---------------------------------------------------------------------------------
+![Figure 3](/assets/images/2019-11-18-security-basics-figure3.png "Reduction of Attack Surface of the Syscall Table. Note that the Senty's Syscall Emulation Layer keeps the Containerized Process from ever calling the Host OS.")
 
 ## Secure-by-default
 
diff --git a/website/blog/2020-04-02-networking-security.md b/website/blog/2020-04-02-networking-security.md
index 78f0a6714..5a5e38fd7 100644
--- a/website/blog/2020-04-02-networking-security.md
+++ b/website/blog/2020-04-02-networking-security.md
@@ -69,13 +69,7 @@ a similar syscall). Moreover, because packets typically come from off-host (e.g.
 the internet), the Host OS's packet processing code has received a lot of
 scrutiny, hopefully resulting in a high degree of hardening.
 
---------------------------------------------------------------------------------
-
-![Figure 1](/assets/images/2020-04-02-networking-security-figure1.png)
-
-Figure 1: Netstack and gVisor
-
---------------------------------------------------------------------------------
+![Figure 1](/assets/images/2020-04-02-networking-security-figure1.png "Network and gVisor.")
 
 ## Writing a network stack
 
diff --git a/website/index.md b/website/index.md
index 95d5d16f0..84f877d49 100644
--- a/website/index.md
+++ b/website/index.md
@@ -3,10 +3,10 @@
     <div class="row">
       <div class="col-md-3"></div>
       <div class="col-md-6">
-        <p>gVisor is an <b>application kernel</b> and <b>container runtime</b> providing defense-in-depth for containers <em>anywhere</em>.</p>
+        <p>gVisor is an <b>application kernel</b> for <b>containers</b> that provides efficient defense-in-depth anywhere.</p>
         <p style="margin-top: 20px;">
+          <a class="btn" href="/docs/user_guide/quick_start/docker/">Quick start&nbsp;<i class="fas fa-arrow-alt-circle-right ml-2"></i></a>
           <a class="btn" href="/docs/">Learn More&nbsp;<i class="fas fa-arrow-alt-circle-right ml-2"></i></a>
-          <a class="btn btn-inverse" href="https://github.com/google/gvisor">GitHub&nbsp;<i class="fab fa-github ml-2"></i></a>
         </p>
       </div>
       <div class="col-md-3"></div>
@@ -19,8 +19,8 @@
 <div class="row">
   <div class="col-md-4">
     <h4 id="seamless-security">Container-native Security <i class="fas fa-lock"></i></h4>
-    <p>By providing each container with its own userspace kernel, gVisor limits
-    the attack surface of the host. This protection does not limit
+    <p>By providing each container with its own application kernel, gVisor
+    limits the attack surface of the host. This protection does not limit
     functionality: gVisor runs unmodified binaries and integrates with container
     orchestration systems, such as Docker and Kubernetes, and supports features
     such as volumes and sidecars.</p>
@@ -43,7 +43,7 @@
     The pluggable platform architecture of gVisor allows it to run anywhere,
     enabling consistent security policies across multiple environments without
     having to rearchitect your infrastructure.</p>
-    <a class="button" href="/docs/user_guide/quick_start/docker/">Get Started &raquo;</a>
+    <a class="button" href="/docs/architecture_guide/platforms/">Read More &raquo;</a>
   </div>
 </div>