doc: Document for Peer Group and Dynamic Neighbor

Signed-off-by: Satoshi Fujimoto <satoshi.fujimoto7@gmail.com>

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+# Dynamic Neighbor
+
+This page explains how to configure the Dynamic Neighbor features.
+Dynamic Neighbor enables GoBGP to accept connections from the peers in specific prefix.
+
+## Contents
+
+- [Prerequisite](#prerequisite)
+- [Configuration](#configuration)
+- [Verification](#verification)
+
+## Prerequisite
+Assumed that you finished [Getting Started](getting-started.md) and learned [Peer Group](peer-group.md).
+
+## Configuration
+
+The Dynamic Neighbor feature requires a peer group setting for its configuration.
+
+```toml
+[global.config]
+  as = 65001
+  router-id = "172.40.1.2"
+
+[[peer-groups]]
+  [peer-groups.config]
+    peer-group-name = "sample-group"
+    peer-as = 65002
+  [[peer-groups.afi-safis]]
+    [peer-groups.afi-safis.config]
+      afi-safi-name = "ipv4-unicast"
+  [[peer-groups.afi-safis]]
+    [peer-groups.afi-safis.config]
+      afi-safi-name = "ipv4-flowspec"
+
+[[dynamic-neighbors]]
+  [dynamic-neighbors.config]
+    prefix = "172.40.0.0/16"
+    peer-group = "sample-group"
+```
+
+By this configuration, peers in `172.40.0.0/16` will be accepted by this GoBGP,
+and the `sample-group` configuration is used as the configuration of members of this dynamic neighbor.
+
+Note that GoBGP will be passive mode to members of dynamic neighbors.
+So if both peers listen to each other as dynamic neighbors, the connection will never be established.
+
+# Verification
+
+Dynamic neighbors are not shown by `gobgp neighbor` command until the connection is established.
+```shell
+$ gobgp neighbor
+Peer AS Up/Down State       |#Received  Accepted
+```
+
+After the connection is established, the neighbor will appear by `gobgp neighbor` command.
+You can see the neighbor config is inherited from the peer group config.
+
+```shell
+$ gobgp neighbor
+Peer          AS  Up/Down State       |#Received  Accepted
+172.40.1.3 65001 00:00:23 Establ      |        0         0
+$ gobgp neighbor 172.40.1.3
+BGP neighbor is 172.40.1.3, remote AS 65002
+  BGP version 4, remote router ID 172.40.1.3
+  BGP state = established, up for 00:00:07
+  BGP OutQ = 0, Flops = 0
+  Hold time is 90, keepalive interval is 30 seconds
+  Configured hold time is 90, keepalive interval is 30 seconds
+  
+  Neighbor capabilities:
+    multiprotocol:
+        ipv4-unicast:	advertised and received
+        ipv4-flowspec:	advertised and received
+    route-refresh:	advertised and received
+    4-octet-as:	advertised and received
+  Message statistics:
+                         Sent       Rcvd
+    Opens:                  1          1
+    Notifications:          0          0
+    Updates:                0          0
+    Keepalives:             1          1
+    Route Refresh:          0          0
+    Discarded:              0          0
+    Total:                  2          2
+  Route statistics:
+    Advertised:             0
+    Received:               0
+    Accepted:               0
+```
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+# Peer Group
+
+This page explains how to configure the Peer Group features.
+With Peer Group, you can set the same configuration to multiple peers.
+
+## Contents
+
+- [Prerequisite](#prerequisite)
+- [Configuration](#configuration)
+- [Verification](#verification)
+
+## Prerequisite
+Assumed that you finished [Getting Started](getting-started.md).
+
+## Configuration
+
+Below is the configuration to create a peer group.
+
+```toml
+[[peer-groups]]
+  [peer-groups.config]
+    peer-group-name = "sample-group"
+    peer-as = 65001
+  [[peer-groups.afi-safis]]
+    [peer-groups.afi-safis.config]
+      afi-safi-name = "ipv4-unicast"
+  [[peer-groups.afi-safis]]
+    [peer-groups.afi-safis.config]
+      afi-safi-name = "ipv4-flowspec"
+```
+
+The configurations in this peer group will be inherited to the neighbors which is the member of this peer group.
+In addition, you can add additional configurations to each member.
+
+Below is the configuration to create a neighbor which belongs this peer group.
+
+```toml
+[[neighbors]]
+  [neighbors.config]
+    neighbor-address = "172.40.1.3"
+    peer-group = "sample-group"
+  [neighbors.timers.config]
+    hold-time = 99
+```
+
+This neighbor belongs to the peer group, so the peer-as is 65001, and ipv4-unicast and ipv4-flowspec are enabled.
+Furthermore, an additional configuration is set, the hold timer is 99 secs.
+
+## Verification
+
+You can see the neighbor configuration inherits the peer group config by running `gobgp neighbor` command.
+
+```shell
+$ gobgp neighbor 172.40.1.3
+BGP neighbor is 172.40.1.3, remote AS 65001
+  BGP version 4, remote router ID 172.40.1.3
+  BGP state = established, up for 00:00:05
+  BGP OutQ = 0, Flops = 0
+  Hold time is 99, keepalive interval is 33 seconds
+  Configured hold time is 99, keepalive interval is 33 seconds
+  
+  Neighbor capabilities:
+    multiprotocol:
+        ipv4-unicast:	advertised and received
+        ipv4-flowspec:	advertised and received
+    route-refresh:	advertised and received
+    4-octet-as:	advertised and received
+  Message statistics:
+                         Sent       Rcvd
+    Opens:                  1          1
+    Notifications:          0          0
+    Updates:                0          0
+    Keepalives:             1          1
+    Route Refresh:          0          0
+    Discarded:              0          0
+    Total:                  2          2
+  Route statistics:
+    Advertised:             0
+    Received:               0
+    Accepted:               0
+```