docs: 📝 Document PrimaryGuild and User.primary_guild

[Paillat-dev]

Summary

fixes #2936

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[pycord-app]

Thanks for opening this pull request!
Please make sure you have read the Contributing Guidelines and Code of Conduct.

This pull request can be checked-out with:

git fetch origin pull/2937/head:pr-2937
git checkout pr-2937

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/2937/head

[plun1331]

This can go under users since it's user related