docs: Migrate to gp-sphinx theme and packages (#66)

## Summary

Migrates gp-libs 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` now uses `merge_sphinx_config()` so shared theme
  behavior, extensions, fonts, and asset workarounds come from
  gp-sphinx instead of local boilerplate.
- Removes vendored docs support files now supplied by shared packages:
  `docs/_ext/sphinx_fonts.py`, custom docs CSS/JS, sidebar/page
  templates, and the local `tests/docs/_ext/test_sphinx_fonts.py`
  harness.
- Removes obsolete mypy overrides tied to the deleted docs module.

## Test plan

- [x] `just test`
- [x] `just ruff`
- [x] `just mypy`
- [x] `just build-docs`

Author: tony