Add columns.metadata.list rpc function

[mathemancer]

Fixes #3640

This adds a ColumnMetaData model to replace the parts we still need from the deprecated Column model, and an RPC function at columns.metadata.list (as exposed in the API) to list its members.

Technical details

This PR also reorganizes the documentation. This was necessary to make the docs work with the columns.metadata namespace. I think the new organization setup is actually easier to understand, though.

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[Anish9901]

This looks great!