Moves the shared docs build steps (apt install, uv sync, __PKG_VERSION__
substitution, pre-build command, cd docs && make) into a composite action
at .github/actions/build-docs. The action operates on whatever is already
checked out, so callers own the checkout step.

The docs.yml reusable workflow is simplified to: checkout, call the
composite action, trigger tskit-site rebuild on main.

This also switches the build command from running jupyter-book directly
to "cd docs && make", so the Makefile (and build.sh) is always the
single entry point for docs builds.