Determine how to deprecate themes

Problem/Motivation

We have opened multiple issues planning to deprecate themes:

1. #2659890: [Plan] Drupal 9 markup and CSS backwards compatibility Will likely introduce a new Stable for Drupal 9 meaning that the Drupal 8 Stable should be deprecated on Drupal 9.0.0.
2. #3050378: Replace Classy with a starterkit theme Plans to replace Classy with a starterkit.
3. #3084814: Deprecate Seven theme Plans to replace Seven with a new admin theme.

These are blocked or will be blocked by the fact that at the moment we don't have any documentation on what is the process for deprecating themes..

Proposed resolution

Document process for deprecating themes.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet