Clarify release process

[bartoszmajsak]

What can be improved

Both commands and the final confirmation message are misleading.

• /release should in fact be called /prepare as it is preparing the release commits and tag to be pushed to master
• final msg should indicate that actual release process is about to start

Gold plating goals

• report back the status to the release PR when jobs are done
• link external PRs

Changes should also be reflected in the docs.

Why

So that we are not flabbergasted anymore?! :D