[michalmaj]

Summary

Add a locally previewable MkDocs documentation site and validate it in the existing CI job.

Why

The guide has grown beyond a single README. A small documentation site now gives readers structured navigation and search while keeping every source page readable directly on GitHub.

Changes

• add MkDocs as a development dependency and configure the built-in theme
• add a site landing page, explicit navigation, and a focused MkDocs chapter
• document local preview, strict builds, troubleshooting, and the updated quality workflow
• build the site with mkdocs build --strict in the existing Quality checks job
• ignore generated site/ output and keep publishing out of scope for this PR

Type of change

• Documentation
• Project configuration
• New feature
• Tests
• CI/CD
• Refactoring
• Bug fix

Checklist

• The change is small and focused
• The pull request title is clear
• I updated documentation if needed
• I added or updated tests if needed
• I ran local checks if applicable

Notes

GitHub Pages deployment is intentionally deferred to a separate 11B decision and pull request.

Local verification:

• uv run pre-commit run --all-files
• uv run rumdl check .
• uv run mkdocs build --strict
• uv run pytest --cov=text_toolkit --cov-report=term-missing
• uv run mypy
• uv run ruff check .
• uv run ruff format --check .
• uv sync --locked --group dev
• uv build --no-sources