Add Collections docs

[georgi]

Summary

• document Collections panel, creating collections, indexing files and using them in chat
• link to the new documentation in the index page

Testing

• npm run lint in web
• npm run typecheck in web
• npm test in web
• npm run lint in apps
• npm run typecheck in apps
• npm test in apps
• npm run lint in electron
• npm run typecheck in electron
• npm test in electron

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https://chatgpt.com/codex/tasks/task_b_6858819b78e0832fa637ee8ad9520fed

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