feat(ci): automated release workflow

[shiftinv]

Summary

only took like... 1.5 years, but I finally got around to it.

This automates ~80% of the current release procedure, leaving only those actions that require elevated perms (due to branch protection) or manual work (writing announcements).
The new and improved:tm: release process can be found here, and for the most part now requires some patience and clicking a few buttons.

This also requires some setup in the repo and on PyPI, which I've already done.

Testing

Testing this fully without actually releasing a new version is pretty much impossible, but you can take a look at the PRs and workflow runs in my test repo:

1. Automatic changelog + version PR: release: v2.99.0 disnake-test/disnake#1
2. Tagging a new version automatically triggers GitHub + PyPI releases: https://github.com/disnake-test/disnake/actions/runs/5499894435
3. If the released version was a x.y.0 release, a dev version PR is created as well: chore: update version to v2.100.0a disnake-test/disnake#2

Future Work

Arguably, some parts of this could be automated further, like the readthedocs step. However, https://xkcd.com/1205/.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• 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, ...)

[shiftinv]

Did another round of testing and fixing (tiny) bugs, this should be good now.
I'll do a trial run with v2.9.1 later, fingers crossed