Pin a welcome post in Discussions Q&A

## Task

Discussions is on with a Q&A form, but the category is empty. A pinned welcome post is the first thing pilots see.

## Scope

Maintainer can pin; a contributor can draft the markdown.

Cover:
- Link README, FAQ, `/demo`, known limitations
- Bugs → issues; questions → this category; security → SECURITY.md
- Point at `good first issue`

## Acceptance criteria

- [ ] Draft in `docs/` or the PR description, ready to paste
- [ ] Maintainer pins it in Q&A after merge

Not a substitute for `.github/DISCUSSION_TEMPLATE/q-a.yml` (already shipped).

Start by reviewing the README, FAQ, /demo, known limitations, and SECURITY.md so the welcome post can link to the project’s existing guidance. Draft the markdown in docs/ or the pull request description, covering where to report bugs, questions, and security issues and pointing to good first issue. Done means the draft is ready to paste and the maintainer can pin it in the Q&A category.

Written by the indexing model from the issue text.