Website: add Q&A sidebar demo media

Fill the "Q&A sidebar" showcase row.

- **File:** `website/assets/showcase-qa.{mp4,gif,png,webp}`.
- **Where:** Showcase section, "Ask the meeting, not your memory".
- **Ratio:** 16:10.
- **Should show:** asking a question against the live transcript and a streamed, grounded answer.

Part of #7.

