Add documentation of proposed new Querying API [draft]

[ephys]

This PR isn't meant to be merged.

This is an attempt at a documentation-first approach to the new querying API. I've attempted to cover all use cases that people have regularly requested.

It's build on top of the Query Builder RFC and the Advanced Literals RFC. It also introduces many breaking changes. I don't expect to release this in v7 (it should probably be its own major release considering the importance of the changes).

It adds the ability to do arbitrary joins & subQueries. Allowing us to lock down the include API to a set of safe, controlled, use cases as users can fall back to joins & subQueries if include doesn't cover their use case.

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