Migrate documentation site to MkDocs + material with GitHub Pages Actions deployment

[haakoek]

Summary
This PR migrates documentation from the current Sphinx-based GitHub Pages setup to a MkDocs-based site with a simpler deployment pipeline and lower maintenance burden.

What changed

1. Added MkDocs configuration in mkdocs.yml
2. Replaced docs workflow with GitHub Pages native deploy actions in docs_pages.yml
3. Updated docs dependencies in pyproject.toml to use:

• mkdocs
• mkdocs-material
• mkdocstrings with python handler
• mkdocs-jupyter
• pymdown-extensions

4. Added new docs content tree in docs_site:

• Home and getting started pages
• Theory section
• API reference pages generated from docstrings
• Notebook pages rendered from existing notebooks
• Small theme/style customization via custom CSS

Why

1. Reduce docs maintenance overhead
2. Improve local authoring experience with fast preview
3. Keep API docs synchronized with source docstrings
4. Support notebooks as first-class documentation pages
5. Use a cleaner, modern Pages deployment path

Validation

1. Local build succeeds with mkdocs build
2. Local preview works with mkdocs serve
3. Workflow is configured to build on PR and deploy on push to main

Notes

1. This PR introduces the new MkDocs site in parallel and does not remove legacy docs scripts/content yet.
2. Follow-up cleanup PR can remove unused Sphinx-specific artifacts after migration is accepted.