Open text documents with lazy session initialization

Given the new approach to load a document state from the file content, we should consider adding a way to initialize an editing session lazy. This could be useful for various cases:

• In a read only document we likely don't need to update as often -> can be a large impact on instances that share a lot of read only files that get accessed frequently
• In rich workspaces we could only lock the file once someone actually starts editing
• Collectives could rather use this instead of requiring to switch between different editor components

We'd probably need:

• Have an API to fetch the document content or yjs state without creating a new session, maybe even fetch steps for read only sessions
• Some less frequent polling/notify push could then be used to check if a user editing joins or some steps were pushed
• Think about how a handover can work from the UX perspective
• Think about how the session can be initialized in the frontend code base

Start by mapping the existing document-state and editing-session APIs in the frontend codebase. Define how read-only content or Yjs state is fetched without a session, how polling or notifications detect editing, and how the UX hands over into an initialized session.