Document release process

[mprazz]

Description of Problem:
The release process for Rasa Open Source is not currently documented publicly, and in the interest of transparency, we'd like to refine and publish it on github

Overview of the Solution:
Go over the documentation we already have internally on Notion, unify it and update it if needed.
Create a doc page, submit it to review.

Existing docs can be found here and here.
Original conversation regarding this issue here.