docs: Migrate to gp-sphinx theme and packages (#9)

## Summary

Migrates libtmux-mcp docs from repo-local Sphinx theme/config/assets to
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 settings, fonts, extension defaults, 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, and sidebar/page
  templates.
- Removes the no-longer-needed `sphinx_fonts` mypy override.

## Test plan

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

Author: tony