[DOCS]: Add RAMPART acronym, examples repo references, and contributing nav fix

[nina-msft]

Description

Two small documentation polish changes for new-user discoverability:

• Spell out the RAMPART acronym ("Risk Assessment & Measurement Platform for Agentic Red Teaming") on the landing page and in the glossary, and add the missing link in the Contributing Quick Nav table.
• Surface the microsoft/rampart-examples companion repo from the three places a new user is most likely to land:
  • Home page Quick Navigation gets a "See a runnable, end-to-end demo" row.
  • Getting Started index gets a third row pointing at the demos (e.g. helpdesk-bot).
  • Quickstart gets a tip admonition near the top and a bullet under "What''s Next" for readers who prefer working code over walkthroughs.

All edits are confined to docs/ markdown; mkdocs build succeeds with no new warnings on the touched files.

Breaking changes

None.

Checklist

• pre-commit run --all-files passes (ran on the changed markdown files; no Python files in the diff)
• Tests added or updated for changes (N/A; documentation-only)
• Documentation updated

[bashirpartovi]

LGTM