Documentation Updates

[plun1331]

Summary

Changes and fixes a lot of stuff in the documentation.

Things I remember doing:

• Documented cogs
• Documented ApplicationCommand exceptions and moved extension errors to discord from ext.commands
• Fixed many typos
• Added guide to index
• Renamed migrating to migrating to v1 to make room for a potential v2 migration guide
• Redid half the sink docs
• Fixed warnings while building docs
• Fixed a lot of broken references
• Notes the message content intent requirement at the top of ext.commands documentation (could be moved?)

You can see a preview at https://pycord-plun1331.readthedocs.io/en/latest/index.html

Checklist

• 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
• 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, ...)

[ooliver1]

oh also id highly recommend using proper commit messages when contributing - or just any time you make a commit message
helps with seeing what each commit did and when

[plun1331]

Suggested changes have been applied

[Dorukyum]

Looks good to me, thank you

[Middledot]

lgtm