Remove `project.documentation_type` from all our serving code, and models

[ericholscher]

I did a quick grep for this and it seems there is not many places where we have logic that's based on this field: #10042

[htech9]

@ericholscher @humitos this is just a friendly heads up to ask if some action need to be done on this PR regarding fact
the referred implementation at https://github.com/readthedocs/readthedocs.org/pull/10042 looks stale for a while now?

[humitos]

Hi @htech9. Yes, there are still work that need to happen before being able to move forward. We detailed it at #10042 (comment)