Purpose

This PR makes several improvements to the docs:

• A security notice at the very top, as recommended by the AI App Templates team
• A streamlined CONTRIBUTING guide to focus on PR contributions, with a new section about translations
• Advanced sections of README.md are their own docs/ pages now, and the TOC for the advanced docs is now viewable in the main README. That way you can single-click from the main README to the advanced doc of your choosing.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[ ] Yes
[X] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by this tutorial
which includes deployment, settings and usage instructions. If text or screenshot need to change in the tutorial,
check the box below and notify the tutorial author. A Microsoft employee can do this for you if you're an external contributor.

[ ] Yes
[X] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[ ] Other... Please describe:

Code quality checklist

See CONTRIBUTING.md for more details.

N/A