Add one and only RELEASING.md

[mkurz]

This is WORK IN PROGRESS.

I merged the two files from https://github.com/playframework/play-meta/tree/main/releasing and a typical RELEASE.md from a repo (like https://github.com/playframework/play-json/blob/main/RELEASING.md).

So in the end we will have three section:

• how to release in general with release-by-tag, which now applies for all repos.
• second section for Play core framework because it is a bit special
• third section for sbt-web plugins (however maybe this will be removed or is not necessary, we will see)

I will improve this doc step-by-step, however I finally want to have this in one file only and we can link to it from the README.
All existing RELEASING.md are totally outdated. Doesn't look nice to have such outedated files in the repos.