feat(docs): Move away from MkDocs to Astro Starlight

[iambriccardo]

Summary

• Replaces the MkDocs/Material docs site with an Astro Starlight docs app hosted from dist via GitHub Pages.
• Moves documentation content into src/content/docs, removes the old MkDocs config and Python docs requirements, and keeps the docs content aligned with current ETL destination/store/event semantics.
• Adds Supabase-inspired docs styling, improved tables, links, code block language labels, responsive Mermaid rendering, search, sitemap, and generated llms.txt outputs.
• Updates the docs workflow to build with Node/npm and deploy with the GitHub Pages artifact/deploy actions instead of mkdocs gh-deploy.

MkDocs removal check

• Removed mkdocs.yaml.
• Removed docs/requirements_docs.txt.
• Removed the old MkDocs custom stylesheet under docs/stylesheets/extra.css.
• Verified no tracked references remain for mkdocs, mkdocs-material, gh-deploy, or the old docs requirements file.
• Removed ignored local leftovers from the old MkDocs build/venv (site/, venv/) from the checkout.

[coveralls]

coverage: 73.108% (+0.05%) from 73.057% — fix-docs into main