docs: migrate documentation from raw Quarto to Great Docs

[chriscarrollsmith]

Summary

• Replace the hand-authored Quarto site with Great Docs using the native user guide flow (user_guide/*.qmd, homepage: user_guide).
• Swap dev dependency from PyPI quarto to great-docs, add great-docs.yml, and update CI to build/deploy great-docs/_site to GitHub Pages.
• Keep README as a normal checked-in file and replace the custom RAG bundle with Great Docs-generated llms.txt / llms-full.txt.

Test plan

• PYTHONPATH=. uv run great-docs build succeeds locally
• uv run ty check . passes
• Verify homepage screenshot and generated diagrams render on the published site after merge
• Confirm GitHub Pages deploy updates https://promptlytechnologies.com/fastapi-jinja2-postgres-webapp/

Closes #205