Implement documentation quality commands

[JayTeeBat]

Summary

• implement the advertised check-docs-format, check-docs-links, and build-docs console commands
• validate local targets and heading fragments, with bounded retries and explicit URL-prefix exclusions for external links
• add a strict MkDocs configuration and make this repository run all three documentation gates in CI
• document the commands, generated-output boundary, and outbound-network behavior

Why

The documentation-only profile declared three quality commands that the package did not provide. That made the example aspirational rather than executable. This change turns those names into a supported command surface and makes the reference repository demonstrate the same contract.

Impact

The package now includes Python Markdown, mdformat, and MkDocs as direct runtime dependencies. check-docs-links performs outbound HTTP(S) requests by default; --no-external disables them, and --exclude-url-prefix records intentional exceptions. build-docs replaces the configured MkDocs output directory.

Validation

• uv sync --locked --all-groups
• uv run pre-commit run --all-files
• uv run ruff format --check .
• uv run ruff check .
• uv run ty check
• uv run pytest (30 passed)
• uv run repo-quality check .
• uv run check-docs-format
• uv run check-docs-links
• uv run build-docs
• uv build