Improve documentation on making releases

[choldgraf]

trafficstars

This updates our releases documentation a little bit, with the following main changes:

• Refactor it so that there's a section that's generic to any repository, and a few specific sections for mystmd and myst-theme. This way we can re-use the same content across other projects in jupyter-book org as well, or move the content to the team compass if we want.
• Add documentation about using github-activity for release notes.
• Add documentation encouraging people to push release notes to the blog, for more discoverability from others.