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

1 files changed, 38 insertions, 62 deletions

diff --git a/g3doc/user_guide/platforms.md b/g3doc/user_guide/platforms.md
index eefb6b222..752025881 100644
--- a/g3doc/user_guide/platforms.md
+++ b/g3doc/user_guide/platforms.md
@@ -1,56 +1,27 @@
-# Platforms (KVM)
+# Changing Platforms
 
 [TOC]
 
-This document will help you set up your system to use a different gVisor
-platform.
+This guide described how to change the
+[platform](../architecture_guide/platforms.md) used by `runsc`.
 
-## What is a Platform?
+## Prerequisites
 
-gVisor requires a *platform* to implement interception of syscalls, basic
-context switching, and memory mapping functionality. These are described in more
-depth in the [Platform Design](../../architecture_guide/platforms/).
+If you intend to run the KVM platform, you will also to have KVM installed on
+your system. If you are running a Debian based system like Debian or Ubuntu you
+can usually do this by ensuring the module is loaded, and permissions are
+appropriately set on the `/dev/kvm` device.
 
-## Selecting a Platform
-
-The platform is selected by the `--platform` command line flag passed to
-`runsc`. By default, the ptrace platform is selected. To select a different
-platform, modify your Docker configuration (`/etc/docker/daemon.json`) to pass
-this argument:
-
-```json
-{
-    "runtimes": {
-        "runsc": {
-            "path": "/usr/local/bin/runsc",
-            "runtimeArgs": [
-                "--platform=kvm"
-            ]
-       }
-    }
-}
-```
-
-You must restart the Docker daemon after making changes to this file, typically
-this is done via `systemd`:
+If you have an Intel CPU:
 
 ```bash
-sudo systemctl restart docker
+sudo modprobe kvm-intel && sudo chmod a+rw /dev/kvm
 ```
 
-## Example: Using the KVM Platform
-
-The KVM platform is currently experimental; however, it provides several
-benefits over the default ptrace platform.
-
-### Prerequisites
-
-You will also to have KVM installed on your system. If you are running a Debian
-based system like Debian or Ubuntu you can usually do this by installing the
-`qemu-kvm` package.
+If you have an AMD CPU:
 
 ```bash
-sudo apt-get install qemu-kvm
+sudo modprobe kvm-amd && sudo chmod a+rw /dev/kvm
 ```
 
 If you are using a virtual machine you will need to make sure that nested
@@ -68,31 +39,22 @@ cause of security issues (e.g.
 [CVE-2018-12904](https://nvd.nist.gov/vuln/detail/CVE-2018-12904)). It is not
 recommended for production.***
 
-### Configuring Docker
-
-Per above, you will need to configure Docker to use `runsc` with the KVM
-platform. You will remember from the Docker Quick Start that you configured
-Docker to use `runsc` as the runtime. Docker allows you to add multiple runtimes
-to the Docker configuration.
+## Configuring Docker
 
-Add a new entry for the KVM platform entry to your Docker configuration
-(`/etc/docker/daemon.json`) in order to provide the `--platform=kvm` runtime
-argument.
-
-In the end, the file should look something like:
+The platform is selected by the `--platform` command line flag passed to
+`runsc`. By default, the ptrace platform is selected. For example, to select the
+KVM platform, modify your Docker configuration (`/etc/docker/daemon.json`) to
+pass the `--platform` argument:
 
 ```json
 {
     "runtimes": {
         "runsc": {
-            "path": "/usr/local/bin/runsc"
-        },
-        "runsc-kvm": {
             "path": "/usr/local/bin/runsc",
             "runtimeArgs": [
                 "--platform=kvm"
             ]
-        }
+       }
     }
 }
 ```
@@ -104,13 +66,27 @@ this is done via `systemd`:
 sudo systemctl restart docker
 ```
 
-## Running a container
+Note that you may configure multiple runtimes using different platforms. For
+example, the following configuration has one configuration for ptrace and one
+for the KVM platform:
 
-Now run your container using the `runsc-kvm` runtime. This will run the
-container using the KVM platform:
-
-```bash
-docker run --runtime=runsc-kvm --rm hello-world
+```json
+{
+    "runtimes": {
+        "runsc-ptrace": {
+            "path": "/usr/local/bin/runsc",
+            "runtimeArgs": [
+                "--platform=ptrace"
+            ]
+        },
+        "runsc-kvm": {
+            "path": "/usr/local/bin/runsc",
+            "runtimeArgs": [
+                "--platform=kvm"
+            ]
+        }
+    }
+}
 ```
 
 [nested-azure]: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/nested-virtualization