Create SimpleChat walkthrough video set

## Summary
Create a short official video set covering deployment, admin configuration, latest features, end-user chat, personal workspace uploads, agents/actions, and developer/customization workflows.

## Scope
- Each video should map to a stable docs page, include current UI labels, and identify prerequisite settings where needed. Good first split: deploy/admin, user/workspaces/chat, agents/actions, developer/custom pages.
- Validate links, screenshots, UI labels, and examples against the current SimpleChat experience.
- Keep tenant-specific secrets, IDs, and environment-specific values out of published examples.

## Acceptance Criteria
- [ ] Documentation or content covers the requested scope clearly.
- [ ] Relevant links, images, and examples are reviewed for accuracy.
- [ ] Any related existing feature documentation is linked instead of duplicated.

## Notes
- Source backlog entry: Walkthrough Video Set
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Docs / Content
- Current anchors: `docs/`, `docs/explanation/features/`, `priv-simplechat-2/videos/`
- Proposed roadmap priority: P2
- Proposed roadmap size: L
- Related: #571 covers general FAQ/tutorial/feature walkthrough content; this issue focuses on a video set.