docs: Migrate to gp-sphinx theme and packages (#541)

## Summary

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

## What changed

- `pyproject.toml` / `uv.lock` replace the repo-managed Sphinx/Furo
  stack with published `gp-sphinx==0.0.1a1` and
  `sphinx-argparse-neo==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 implementation now shipped by shared packages:
  local argparse helpers, the embedded `sphinx_argparse_neo` package,
  `sphinx_fonts.py`, custom docs CSS/JS/templates, and related docs test
  coverage.
- Adds follow-up docs lexer typing cleanups so the default `just mypy`
  target covers the docs extension modules cleanly.

## Test plan

- [x] `env -u VIRTUAL_ENV just test` -- 957 passed
- [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,
  65 warnings

Author: tony