more doc (gridstack#3124)

* 📚 Fix documentation sync: Add doc/html and automation

- Manually synced doc/html from master to gh-pages branch
- Added GitHub Actions workflow to auto-sync docs when updated
- Added workflow to auto-generate docs when source changes
- Updated .gitignore to handle .DS_Store files

Fixes: gh-pages doc/html not being automatically copied from master

The doc/html directory now exists on gh-pages and will be automatically
kept in sync with master via GitHub Actions workflows.

* 🔗 Fix API documentation link

Updated index.html to point to correct documentation path:
- Changed from docs/html/index.html to doc/html/index.html
- Ensures the API link in the navigation works correctly
- Now matches the actual location of the synced documentation

* 🔧 Fix Angular documentation path and sync content

- Updated index.html to point to correct Angular docs path (angular/doc/html)
- Synced Angular documentation from master to angular/doc/html
- Removed redundant docs/ and angular/docs/ directories
- Ensured consistent documentation structure across both main and Angular libs

Both main library (doc/html) and Angular library (angular/doc/html)
documentation are now correctly positioned and will sync from master.

Author: adumesny