Release guide in the docs

[webknjaz]

This is a task for "after release". Here's how I've been organizing release guides elsewhere:

• https://github.com/cherrypy/cheroot/blob/fdaa24/docs/contributing/release_guide.rst / https://cheroot.cherrypy.dev/en/latest/contributing/release_guide/
• https://github.com/ansible/pylibssh/blob/bf0528d/docs/contributing/release_guide.rst / https://ansible-pylibssh.rtfd.io/en/latest/contributing/release_guide/
• https://github.com/ansible/awx-plugins/blob/db2cae7/docs/contributing/release_guide.md / https://awx-plugins-core.readthedocs.io/en/latest/contributing/release_guide/
• https://github.com/aio-libs/aiohttp/blob/4296730/docs/contributing-admins.rst / https://docs.aiohttp.org/en/latest/contributing-admins.html#creating-a-new-release

At this point, it's too early to settle on the release process, though. I'd like to make it more automatic, like I did on the projects with workflow_dispatch. But that would need more integration. I propose that we observe and record anything that's happening during this next release so we could have a bird-eye view on what needs to be documented and what should change.

We'll probably deviate from https://jazzband.co/about/releases#continuous-integration by quite a bit.