Missing dicussion of External Proposals and validating incoming External Commits

I would take sections: "8. Joining via Annotated Welcome" and "9. Joining via External Commit" and stick them under a new section:

```
# Joining a group

## Joining via Annotated Welcome

## Joining via External Proposal

Partial MLS clients can join a room by sending an External Proposal, with the `new_member_proposal` SenderType, as described in {{Section 12.1.8 of !RFC9420}}. If the external add proposal is accepted by a member of the group, the Partial MLS client will receive an Annotated Welcome. [Maybe add some text about detecting that the group can deliver Annotated Welcomes before sending the External Proposal].

## Joining via External Commit
```

Then I would add a short section under "Annotated Commit" called "Annotated External Commit", and describe what is different (resolution_index maybe?).

Start by locating the document sections titled "8. Joining via Annotated Welcome" and "9. Joining via External Commit", then read Section 12.1.8 of RFC9420 for External Proposals. Reorganize the joining material under a new "Joining a group" section, add the External Proposal content, and document the differences for an "Annotated External Commit", including resolution_index if applicable.

Written by the indexing model from the issue text.