docs: Migrate to gp-sphinx theme and packages (#345)

## Summary

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

## What changed

- `pyproject.toml` / `uv.lock` switch the docs stack to
  `gp-sphinx==0.0.1a1` and `sphinx-argparse-neo==0.0.1a1` from PyPI.
- `docs/conf.py` is already rewritten around `merge_sphinx_config()` so
  theme settings, shared extensions, fonts, and JS workarounds come from
  gp-sphinx instead of local boilerplate.
- Adds root doctest support for `monkeypatch` and fixes the
  `_colors.py` examples so they match the actual `NO_COLOR` precedence
  contract.
- Retains the removal of vendored docs-only files and now-unused mypy
  overrides from the original gp-sphinx migration stack.

## Test plan

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

Author: tony

conftest.py

@@ -0,0 +1,18 @@
+"""Root pytest configuration for cihai-cli."""
+
+from __future__ import annotations
+
+import typing as t
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _doctest_monkeypatch(
+    doctest_namespace: dict[str, object],
+) -> t.Iterator[None]:
+    """Expose monkeypatch in source doctests and undo env changes after use."""
+    monkeypatch = pytest.MonkeyPatch()
+    doctest_namespace["monkeypatch"] = monkeypatch
+    yield
+    monkeypatch.undo()