[docs] New documentation page State

[flaviendelangle]

Closes #3144
Doc preview

• New "State" documentation page to document how to use the selectors and the initialState prop
• New docs:api:build script to generate a JSON with all the selectors
• Explicit warning when trying to use useGridSelector before state initialization instead of just letting the code crash

For Pagination / Filter / Sorting selectors:

• New auto-generated Selectors section on the doc page
• JSDoc description for every selector and @ignore for selectors we don't want to expose publicly

I think we should deprecate the ill-named selectors or selectors to be removed before documenting them.
So that the selector list on the documentation is clean.
For instance selection and columns selector have not yet be renamed correctly.
If there are selectors you think we should not document, tell me so. I don't document top level selectors like gridFilterStateSeledctor because I would like to stop exporting them from their file. To only expose the interesting values, not the whole sub-filter.

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I'm not a huge fan of the UI for the selector collection.
But I think the apiRef collection and events collection are not great either and that we need to rethink those 3 UIs to have something coherent.

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