Pin a welcome post in Discussions Q&A #18

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).