docs: Migrate to gp-sphinx theme and packages (#52)

## Summary

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

## What changed

- `pyproject.toml` / `uv.lock` replace the manual 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 JS workarounds come
  from gp-sphinx.
- Removes vendored docs implementation now shipped by shared packages:
  `docs/_ext/argparse_*`, `docs/_ext/cli_usage_lexer.py`, the embedded
  `docs/_ext/sphinx_argparse_neo/` package, `docs/_ext/sphinx_fonts.py`,
  custom docs CSS/JS/templates, and the corresponding `tests/docs/_ext/`
  coverage.
- Removes obsolete mypy overrides for the deleted local docs modules.

## Test plan

- [x] `make test`
- [x] `make ruff`
- [x] `make mypy`
- [x] `make build_docs`

Author: tony