docs: Document context servers (#21170)

This PR adds documentation for context servers.

Release Notes:

- N/A

docs/src/assistant/context-servers.md

@@ -0,0 +1,49 @@
+# Context Servers
+
+Context servers are a mechanism for pulling context into the Assistant from an external source. They are powered by the [Model Context Protocol](./model-context-protocol.md).
+
+Currently Zed supports context servers providing [slash commands](./commands.md) for use in the Assistant.
+
+## Installation
+
+Context servers can be installed via [extensions](../extensions/context-servers.md).
+
+If you don't already have a context server, check out one of these:
+
+- [Postgres Context Server](https://github.com/zed-extensions/postgres-context-server)
+
+## Configuration
+
+Context servers may require some configuration in order to run or to change their behavior.
+
+You can configure each context server using the `context_servers` setting in your `settings.json`:
+
+```json
+{
+  "context_servers": {
+    "postgres-context-server": {
+      "settings": {
+        "database_url": "postgresql://postgres@localhost/my_database"
+      }
+    }
+}
+```
+
+If desired, you may also provide a custom command to execute a context server:
+
+```json
+{
+  "context_servers": {
+    "my-context-server": {
+      "command": {
+        "path": "/path/to/my-context-server",
+        "args": ["run"],
+        "env": {}
+      },
+      "settings": {
+        "enable_something": true
+      }
+    }
+  }
+}
+```