Document release process

[Jackenmen]

Preview: https://red-devguide--1.org.readthedocs.build/core-devs/release-process/

[Jackenmen]

The descriptions should be correct enough here.

There are still some improvements to be made here though:

• add some screenshots to step 9 (environment run)
• step 3 is missing a bit of information on where the changelog PR should be merged
  • it should be merged into V3/develop, backported into 3.x if that's the branch you make a release from, cherry-picked into the short-lived branch in step 6
• it might be worth converting the stuff done by my RedHelper cog to a simple Python script so that we don't depend on my bot running
  • on the bright side, the getcontributors and getunreleasedcommits commands are available to be run by everyone, not just the bot owner
  • getunreleasedcommits does not work well with 3.x branch as it checks against V3/develop
• more information on milestone management might be needed, especially with the regular and backport PRs being separate (cherry-pick bot is not yet done so that's also a bit of an issue :))