docs: add self-hosting guide

[Uarmagan]

Summary

Adds the first pass of Compass self-hosting documentation and clarifies the difference between local development setup and product hosting/account modes.

This PR is docs-only. It intentionally does not implement the future one-command installer; that will happen in a follow-up branch.

What changed

• Added a short README entry point for self-hosting.
• Added docs/self-hosting.md for people manually setting up Compass today.
• Added docs/development/hosting-modes.md for contributors to understand hosting context, account state, and data ownership.
• Renamed docs/development/env-and-dev-modes.md to docs/development/local-development.md so local developer setup is not confused with hosting modes.
• Updated docs index and onboarding links to use the new names.
• Added docs/superpowers/ to .gitignore so future brainstorming/design drafts do not get committed by default.

Why

We identified two separate audiences that needed different docs:

• People self-hosting Compass need practical setup guidance: what services they provide, what Compass provides, how to start the app, and where their data lives.
• Contributors need a mental model for how Compass behaves across anonymous browser use, self-hosted account mode, and hosted Compass Cloud.

Splitting those concerns keeps the public self-hosting guide approachable while still giving contributors the deeper map they need.

Important behavior captured

• Anonymous users can use Compass without an account; events and tasks stay in browser IndexedDB.
• After signup, local events migrate to the MongoDB configured for that backend.
• Tasks currently stay in IndexedDB even after signup.
• Self-hosted account mode needs MongoDB and SuperTokens.
• Google Calendar sync is optional as a product behavior, but the backend currently still requires Google env values at startup.
• Server deployments need public web/API URLs, HTTPS, correct FRONTEND_URL / BASEURL / CORS, and MongoDB/SuperTokens reachable from the backend.

Validation

• Checked local markdown links in the touched docs.
• Reviewed the branch diff against main to ensure this branch is docs-focused.
• Moved the work from the earlier misleading feat/one-cmd-dev-setup branch to docs/add-self-hosting-docs; the actual installer work will be handled separately.

Follow-up work

• Make Google config optional for password-only self-hosted account mode: Make Google configuration optional for self-hosted account mode #1659.
• Build the future one-command self-host installer.
• Add deeper self-hosting guides for Google Cloud setup, MongoDB setup/backups, SuperTokens setup, and server deployment.