Add doc tests (pytest-markdown-docs) to template

[JnyJny]

Port the doc testing setup from busylight-core to the cookiecutter template so all new projects get it out of the box.

What to add:

• pytest-markdown-docs in dev dependencies
• docs/conftest.py scaffold with pytest_markdown_docs_globals() hook
• poe doc-test task: pytest --markdown-docs docs/ --ignore=docs/gen_ref_pages.py
• Include doc-test in poe qc sequence
• CI workflow: add poe doc-test step alongside unit tests (both ci.yml and release.yaml)
• CONTRIBUTING.md and CLAUDE.md sections explaining doc tests, continuation/notest markers
• Auto-merge on LGTM workflow (auto-merge.yml)
• Branch protection setup notes in README or CONTRIBUTING

Reference implementation: JnyJny/busylight-core PR #47 and #48.

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Authored by Jny