## Summary

- Adds a new concept page (`docs/src/windows-and-projects.md`)
explaining how Zed handles multiple projects in windows
- Fixes the VS Code migration guide which incorrectly stated "Zed does
not support multi-root workspaces"
- Adds cross-links from Getting Started, CLI Reference, and Parallel
Agents docs

## Context

With multi-workspace now universally enabled, the default behavior is:
- **File > Open** opens projects in the current window's sidebar (not a
new window)
- **Cmd+Enter** in Open Recent opens in a new window
- **CLI `-n` flag** forces a new window

The VS Code migration guide was telling users the opposite of reality.
This PR fixes that and provides a central concept page that other docs
can link to.

## Changes

| File | Change |
|------|--------|
| `windows-and-projects.md` | New concept page |
| `SUMMARY.md` | Add to Working with Code section |
| `getting-started.md` | Note about sidebar default + link |
| `migrate/vs-code.md` | Fix "no multi-root" claim → explain sidebar
model |
| `reference/cli.md` | Link to concept page from flags section |
| `ai/parallel-agents.md` | Cross-reference in Multiple Projects section
|

## Test plan

- [ ] Verify page renders correctly on docs site
- [ ] Check all internal links resolve
- [ ] Confirm `{#kb projects::OpenRecent}` renders the correct
keybinding

Release Notes:

- N/A