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

Problem/Motivation

Per #3050378: [meta] Replace Classy with a starterkit theme the Classy theme is being deprecated in #3307241: Deprecate Classy in favor of the new Starterkit Theme and will be removed in Drupal 10.

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

Proposed resolution

Do the work necessary to remove Classy from core.

Remaining tasks

1. ✅ Do a final subtree split of core/themes/classy from the 10.0.x branch into the contrib Git repo at https://www.drupal.org/project/classy. #3308540: Split the latest core classy history into the 1.0.x branch and cut a 1.0.0 release
2. ✅ Release Classy 1.0.0 stable release
3. ✅ Open an issue against https://www.drupal.org/project/project_composer. #3302966: Ensure that classy doesn't get special core treatment
4. ✅ Manually test switching from the core theme to the contrib theme. #3309319: Manually test upgrade core Classy to contrib Classy
5. ✅ Remove Classy theme from core. #3110137: Remove Classy from core
6. Triage and move core issues for Classy theme from the core queue to the contrib module. #3332482: [Meta] Classy issue triage
7. ✅ Move all core documentation for Classy theme to the contrib project.
8. ✅ When the project_composer step is Fixed, update the documentation with the new composer command.

User interface changes

Classy will be removed from core.

API changes

None.

Data model changes

None.

Release notes snippet

Needed.