docs(chore): Remove bundled extensions and migrate conf.py to gp-sphinx

why: sphinx_fonts, templates, static assets, and spa-nav.js are now
     provided by gp-sphinx packages. Upstream extension tests are
     covered by the PyPI packages.
what:
- Delete docs/_ext/sphinx_fonts.py
- Delete docs/_static/css/custom.css, js/spa-nav.js
- Delete docs/_templates/ (page.html, sidebar/brand.html, sidebar/projects.html)
- Delete tests/docs/ entirely
- Rewrite docs/conf.py to use merge_sphinx_config() and make_linkcode_resolve()

Author: tony