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.
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
- Keep docs/_ext/fastmcp_autodoc.py, docs/_static/js/prompt-copy.js,
  docs/_templates/book.html (project-specific)
- Rewrite docs/conf.py to use merge_sphinx_config() and make_linkcode_resolve()

Author: tony