Prepare repository for new CSS-AAM document

[ZoeBijl]

During TPAC 2025 it was decided to restart work on CSS-AAM. The spec itself will be in the ARIA monorepo. This repository will be for issue tracking and the github-pages. We need to clean this repository and update information to make that happen.

To do

• remove unused files from main (pending in chore: prepare repository for new CSS-AAM #12)
• remove unused files from gh-pages (pending in chore: clean out gh-pages #13)
• remove unused branches
• clean out issues
• clean out pull requests