feat: add Sphinx documentation scaffolding to template

[mariushelf]

Summary

Vendors the documentation scaffold (from the update-docs skill's assets) into
the copier template so that new projects start with a working Sphinx + MyST
documentation site — organised by audience (orientation / users / operators /
maintainers), with only docs/source/ rendered, plus build targets, a docs
dependency group, and a drift-tripwire test harness.

This implements the doc-tooling/HANDOVER-copier-template.md work order.

What lands in a generated project

Source asset	Lands at
docs-source-skeleton/ (whole tree)	docs/source/
meta layer (voice.md, documentation_guide.md, writing_documentation.md)	docs/source/contributing/
makefile-targets.mk	appended to Makefile
docs dependency group	merged into pyproject.toml [dependency-groups]
tests-docs-harness/	tests/docs/

Templating

Only files needing substitution carry a .jinja suffix (copier renders content
only for suffixed files; everything else — including _templates/autosummary/module.rst,
which is itself Jinja meant for Sphinx — is copied verbatim):

• conf.py.jinja — PROJECT_NAME = project_name, DIST_NAME =
  project_slug | replace('_','-') (the PyPI/distribution name, distinct from
  the human title on purpose), AUTHOR = author_name.
• index.md.jinja — title + intro from project_name / project_short_description.
• reference/python-api.md.jinja — autosummary root = project_slug.

The meta layer is vendored byte-identically (it already carries a provenance
header pointing back to the canonical copy in the skill's assets/meta/); it is
never forked, so scaffolded repos stay consistent with what the check-docs
audit expects.

CI / verification

• The generated project's CI (cicd.yaml) gains a docs job: make docs-strict
  (Sphinx -W, warnings = errors) + make test-docs.
• The template's own test suite (tests/test_template.py) now asserts a freshly
  generated project: renders the scaffold with placeholders resolved, builds with
  make docs-strict, and passes make test-docs.

Verified locally end-to-end — a generated project builds docs with zero Sphinx
warnings, and passes make test-docs, make lint, make test, and
pre-commit over the full tree. Full template suite: 9 passed.

Note

The vendored tests/docs/test_doc_claims.py harness needed two small lint
adaptations to satisfy this template's ruff config (a D205 single-line summary,
and moving a # noqa: F401 out of a commented-out example to avoid RUF100).
Worth backporting to the upstream skill asset so other scaffolded repos don't hit
the same.

🤖 Generated with Claude Code