Add "common structures" heading to rendezvous structures docs

Striven

2025-09-09 f723e756d80d68a33917f56ffcc743fc3323353d

 04_rendezvous/01_protocols/01_loginprotocol.md

@@ -67,6 +67,9 @@

### Successful Response

> [!NOTE]
> TODO: Example

| Name                 | Description                                                        | Type                                             |
|----------------------|--------------------------------------------------------------------|--------------------------------------------------|
| User PID             | ID of the user profile that the connected client is identified as. | [`UUID`](../03_rendezvous_structure.md#uuid)     |