[Meta] Tasks to remove Stable theme from core and move to contrib

Problem/Motivation

This issue is to coordinate and track the steps needed to remove stable from core to contrib. See Remove a core module and move it to a contributed project of the deprecation policy.

Proposed resolution

n.a.

Remaining tasks

All done!

Completed tasks

1. Create a child issue, 'Remove Stable from core'. #3309176: Remove Stable theme from Drupal 10
   • The change record for this issue should include a link to recommendations page, https://www.drupal.org/node/3223395#s-EXTENSION_NAME. (For example, the CR for removing HAL)
   • Tag this issue 'Needs release note.'
   • Remove the extension ;-).
   • Remove references from core/phpstan-baseline.neon.
   • Check for references in @todo.
2. Make sure Stable contrib uses drupal/stable as namespace #3302964: Ensure that stable doesn't get special core treatment
3. Move all core documentation for Stable to the contrib project.
4. Manually test upgrading from the core theme to the contrib theme. #3309315: Manually test upgrade core Stable to contrib Stable
5. When the issue in https://www.drupal.org/project/project_composer is Fixed update the documentation with the new composer command.
6. Triage issues in the core queue and move either to the contrib project or the relevant core component. #3332485: [Meta] Stable issue triage

User interface changes

API changes

Data model changes

Release notes snippet