fix: 🐛 Fix docs rendering for ForwardedMessage and MessageSnapshot and add missing attributes

[ToothyDev]

Summary

This PR makes it so the docs for ForwardedMessage and MessageSnapshot actually render in the docs, as well as initialising the documented ForwardedMessage.mentions and ForwardedMessage.type property, see #3050

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!
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This pull request can be checked-out with:

git fetch origin pull/3051/head:pr-3051
git checkout pr-3051

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3051/head

[ToothyDev]

The length of data["mentions"] was 0 whenever the forwarded message came from either DMs or a different server, so for some reason this only seems to be able to be populated when message is forwarded within a server. Not sure if this is a Discord limitation, but seems so.

Should this be part of the docstring?

[Paillat-dev]

lgtm

[Paillat-dev]

tested, works; code lgtm