Add BGP Confederation documentation

Fix #2146

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+# BGP Confederation
+
+This page explains how to configure BGP confederation feature when BGP peers
+are part of a larger mesh representing a single autonomous system (AS).
+
+## Prerequisites
+
+Assume you finished [Getting Started](getting-started.md).
+
+## Contents
+
+- [Configuration](#configuration)
+
+## Configuration
+
+If AS30 is a confederation composed of AS65001 and AS65002, the confederation members must configure
+the following attributes to ensure GoBGP communicates in the correct manner with other member ASNs.
+Each confederated autonomous systems must configure the `[global.confederation.config]` with
+`enabled = true` and `identifier = 30`. The identifier parameter is used to designate what the
+confederation should present as it's ASN non-confederation members. Each member of the confederation
+must also configure `member-as-list` with a list of other ASNs which compose the confederation. For
+example, AS65001 would configure this attribute as `member-as-list = [ 65002 ]`.
+
+```toml
+[global.config]
+as = 65001
+router-id = "10.0.0.1"
+  [confederation.config]
+  enabled = true
+  identifier = 30
+  member-as-list = [ 65002 ]
+
+[[neighbors]]
+  [neighbors.config]
+    peer-as = 65002
+    neighbor-address = "10.0.0.2"
+```