Adds documentation for the Parallel Agents feature.

## Changes

- **New page** `docs/src/ai/parallel-agents.md`: covers the Threads
Sidebar component, switching threads, the archive and search, importing
ACP threads, running multiple threads, multiple projects, worktree
isolation, and the default layout change
- **`docs/src/ai/overview.md`**: rewrites the Agentic editing section to
lead with the Threads Sidebar, adds Parallel Agents to the Getting
started links
- **`docs/src/ai/agent-panel.md`**: adds a cross-link to the Threads
Sidebar in Creating New Threads, removes the Navigating History section
(now covered in the Threads Sidebar docs)
- **`docs/src/SUMMARY.md`**: adds Parallel Agents between External
Agents and Inline Assistant

Release Notes:

- N/A

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Co-authored-by: Katie Geer <katie@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>