Adds API docs for "POST /docs"

[Spoffy]

Adds API documentation for POSTing to the "/docs" endpoint.

The /docs endpoint has four behaviours:

1. Copy an existing document
2. Upload a new document from a file
3. Create a new document in a specific workspace
4. Create a new unsaved document

Each documented using its own schema.

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[Spoffy]

This overlaps with this endpoint: /workspaces/{workspaceId}/docs, which creates a new, saved doc in the given workspace.

This means it looks odd in the API documentation, having two "Create new doc" endpoints:

My thinking is we avoid documenting use cases 3. and 4. (creating [un]saved documents), and document this purely as a Copy/Upload endpoint.

Any thoughts would be appreciated!

[Spoffy]

On hold until the underlying API can be de-overloaded