Improve agent documentation and walkthrough guidance

@paullizer is already working on this.

Since Jul 2, 2026.

## Summary
Improve SimpleChat's agent documentation so users and admins can understand what agents are, how agents are used, and how to choose, configure, and work with them effectively.

## Scope
- Explain what agents are in SimpleChat, including their role in chat workflows, tools/actions, prompts/instructions, and workspace contexts.
- Document common agent usage patterns for end users and admins, including selecting agents, configuring agents, and understanding when to use an agent versus standard chat.
- Add or improve agent-related docs pages, descriptions, walkthroughs, and references from existing documentation entry points.
- Identify and link relevant video content or create descriptions/outlines for agent walkthrough videos where video assets are planned or available.
- Clarify related terminology such as agents, actions/tools, workflows, knowledge sources, and workspace scope.

## Acceptance Criteria
- [ ] Agent documentation includes a clear "What are agents?" explanation for non-expert users.
- [ ] Documentation explains how agents are used from both user and admin perspectives.
- [ ] Agent setup, selection, and configuration guidance is linked from the main docs navigation or relevant landing pages.
- [ ] Agent docs include practical examples or walkthrough scenarios.
- [ ] Video descriptions, outlines, or links are added where agent video content exists or is planned.
- [ ] Related existing docs are reviewed for consistency and updated to point to the improved agent documentation.

## Notes
This is a documentation-focused improvement; no product behavior changes are expected unless gaps are discovered while documenting.

Related but not duplicate:
- #943 tracks a broader SimpleChat walkthrough video set.
- #959 tracks admin walkthroughs for common agent templates.