feat: ✨ Add docs on shared inboxes

app/federation/page.mdx

@@ -31,6 +31,12 @@ To perform this action, Alice's instance sends a `POST` request to Bob's inbox e
 
 Bob's instance receives the message and processes it according to the rules defined in the [Validation](/federation/validation) document.
 
+### Shared Inboxes
+
+In addition to personal inboxes, instances can also have shared inboxes. Shared inboxes are a single per-instance endpoint that can receive messages for multiple users. This is useful for instances that want to reduce the number of HTTP requests they need to handle.
+
+Shared inboxes are defined in the [Instance Metadata](/entities/instance-metadata) entity. If supported, they should be used in place of personal inboxes.
+
 ## Outboxes
 
 In addition to inboxes, every user has an outbox (e.g., `/users/3/outbox`). The outbox is simply a collection of all the messages that a user has sent. When a user sends a message to another user, a copy of that message is accessible in the sender's outbox.