Add create document api endpoint

[sampaccoud]

Purpose

We want trusted external applications to be able to create documents via the API on behalf of any user.

Proposal

Add an action API endpoint limited to document creation and with a basic server-to-server authentication:
/api/v1.0/documents/create-for-user/

Expected payload:

{
    "title": "My document title",     # document title 
    "content": "My document content", # document content in ydoc format
    "sub": "123",                     # "sub" of the user in the remote application who may pre-exist in our db
    "email": "john.doe@example.com",  # email to send invitation in case the user "sub" doesn't exist in our db
    "language": "fr-fr",              # [optional] language in which the invitation email should be sent
    "message": "My custom message",   # [optional] custom message in the invitation email
    "subject": "My custom subject",   # [optional] custom subject for the invitation email
}

The user may or may not pre-exist in our database and should be notified of the document creation by email.

[lebaudantoine]

This endpoint breaks encapsulation by requiring external services to pass data in the highly specific Yjs format, which is tightly coupled to the Doc implementation. It impacts the usability of the endpoint.

Moreover, it should implement a robust validation to ensure the content value is well-formed and will not cause errors when opening the document.

Finally, allowing the conversion of Markdown strings into Yjs format, supporting broader use cases like importing or transforming existing documents, would be a really nice enhancement.

Some of these requirements depend on the ability to execute JavaScript code. This capability is expected to be available soon, once @AntoLC completes the upgrade to the collaboration service.

We agreed to merge my draft microservice into the collaboration one.

This endpoint may need to adapt its behavior based on the input format. Should we consider adding a parameter to explicitly specify the format?

This PR also introduces documents without any owner, which might need to be discussed.

I really need this endpoint ASAP. Should we merge it as proposed initially by @sampaccoud, and iterate with other PR?

[sampaccoud]

All nitpicks. Move ahead!