docs: add Factory Desktop getting-started docs

[factory-sagar]

Summary

Add GA getting-started documentation for Factory (Desktop) under /desktop/... (Overview + Quickstart), and update Welcome + sidebar navigation to surface Desktop alongside CLI.

Changes

• New pages
  • docs/desktop/getting-started/overview.mdx
  • docs/desktop/getting-started/quickstart.mdx
• Navigation
  • docs/docs.json: add Desktop group under Welcome (replacing the previous App/Web entries)
• Welcome
  • docs/welcome/index.mdx: replace the Web card with a Desktop card linking to /desktop/getting-started/overview
• Assets
  • docs/images/desktop/prompt-settings.png
  • docs/images/desktop/add-mcp-server.png

Notes

• Desktop overview includes Mission Mode and Native MCP Support (MCP registry wording).

Missing / follow-ups

• Desktop Overview: still needs the correct hero/product image asset.
• Desktop Quickstart → Chat Input: screenshot needs updating to the correct/production UI (no dev path; ~/.factory/mcp.json instead of ~/.factory-dev/mcp.json).

Validation

• python3 -m json.tool docs/docs.json
• Checked that all docs/docs.json page references resolve to existing files

[factory-sagar]

Note: this PR is still missing the right image assets for (1) Desktop Overview hero/product image and (2) Desktop Quickstart “Chat Input” screenshot (should reflect production UI; ~/.factory/mcp.json not ~/.factory-dev/mcp.json).

[factory-droid]

Desktop docs additions look correct overall, and navigation updates point to the new pages. No high-severity issues (P0/P1) found in this diff.