docs: Migrate to gp-sphinx theme and packages (#519)

## Summary

Migrates libvcs docs from repo-local Sphinx theme/config/assets to the
published `gp-sphinx` shared documentation platform.

## What changed

- `pyproject.toml` / `uv.lock` replace the repo-managed Sphinx + Furo
  dependency stack with `gp-sphinx==0.0.1a1` from PyPI.
- `docs/conf.py` is rewritten around `merge_sphinx_config()` so shared
  theme defaults, extensions, fonts, and asset workarounds come from
  gp-sphinx.
- Removes vendored docs-only files now provided by shared packages:
  `docs/_ext/sphinx_fonts.py`, custom docs CSS/JS, sidebar/page
  templates, and local docs test harness code.
- Removes unused mypy overrides tied to deleted docs modules.

## Test plan

- [x] `env -u VIRTUAL_ENV just test` -- 586 passed, 1 skipped
- [x] `env -u VIRTUAL_ENV just ruff` -- all checks passed
- [x] `env -u VIRTUAL_ENV just mypy` -- no issues found
- [x] `env -u VIRTUAL_ENV just build-docs` -- build succeeded,
  3 warnings

Author: tony