chore: switch docs build to PyPI sphinx-pyrunblock instead of git+https (#221)

## Summary

`sphinx-pyrunblock` is Peter's own package (a hard fork of the original
`WhyNotHugo/sphinx-autorun`, published properly to PyPI under this
name). Today's CI installs it via `pip install
git+https://github.com/petercorke/sphinx-autorun.git` -- an old,
pre-rename URL that still works only because GitHub redirects renamed
repos. This switches to installing the plain PyPI wheel instead.

- Bumps `pyproject.toml`'s `docs` extra from `"sphinx-autorun"` to
`"sphinx-pyrunblock"` (the old entry was misleading in isolation --
without the workflow's follow-up git install, it would resolve to the
unrelated upstream PyPI package of the same name, not Peter's fork).
- Drops the now-redundant `pip install git+...` line from `sphinx.yml`,
since `pip install .[dev,docs]` already installs the right package.
- Updates `conf.py`'s `extensions` list from `sphinx_autorun` to
`sphinx_pyrunblock` to match.

## No functional change

Both the old git URL and the new PyPI package resolve to the identical
`sphinx-pyrunblock==1.1.0` release today -- confirmed with a fresh,
uncached `pip install git+...` and comparing installed package metadata.
This is purely a speed/hygiene change.

## Why

The PyPI install is consistently 2-4x faster in a clean venv (no git
clone/VCS metadata resolution):

| Method | Run 1 | Run 2 |
|---|---|---|
| `git+https://...sphinx-autorun.git` | 20.5s | 10.6s |
| `sphinx-pyrunblock` (PyPI) | 5.4s | 7.1s |

That's ~9-15s saved on every docs-build CI run (every push to `master`
plus every PR).

## Test plan

- [x] Fresh isolated venv, `pip install ".[dev,docs]"`, confirmed
`sphinx-pyrunblock==1.1.0` resolves without the git install line
- [x] Real `sphinx-build -b html` against the rebased branch (includes
#198, #162) -- succeeds, 15 warnings, consistent with pre-change
baseline

Co-authored-by: Tim Perkins <code@taughz.dev>

Author: petercorke

.github/workflows/sphinx.yml

@@ -16,7 +16,6 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install .[dev,docs]
-        pip install git+https://github.com/petercorke/sphinx-autorun.git
         pip install sympy
         sudo apt-get install graphviz
     - name: Build docs

docs/source/conf.py

@@ -47,7 +47,7 @@
     "sphinx.ext.inheritance_diagram",
     "matplotlib.sphinxext.plot_directive",
     "sphinx_autodoc_typehints",
-    "sphinx_autorun",
+    "sphinx_pyrunblock",
     "sphinx.ext.intersphinx",
     "sphinx_favicon",
 ]

pyproject.toml

@@ -61,7 +61,7 @@ dev = [
 docs = [
     "sphinx",
     "sphinx-rtd-theme",
-    "sphinx-autorun",
+    "sphinx-pyrunblock",
     "sphinxcontrib-jsmath",
     "sphinx-favicon",
     "sphinx-autodoc-typehints",