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

why: sphinx_fonts, argparse extensions, 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/ bundled extensions (sphinx_fonts, argparse_*, sphinx_argparse_neo/)
- Keep docs/_ext/vcspull_console_lexer.py, vcspull_output_lexer.py (project-specific)
- Delete docs/_static/css/custom.css, css/argparse-highlight.css, js/spa-nav.js
- Delete docs/_templates/ (page.html, sidebar/brand.html, sidebar/projects.html)
- Delete tests/docs/ upstream extension tests
- Keep tests/docs/_ext/test_vcspull_*_lexer.py (project-specific)
- Rewrite docs/conf.py to use merge_sphinx_config() and make_linkcode_resolve()

Author: tony