Add docs for custom provider fields and query parameters

[ml-evs]

Closes #1123.

[codecov]

Codecov Report

Merging #1164 (a42bdf7) into master (255a402) will not change coverage.
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  Coverage   92.32%   92.32%           
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[JPBergsma]

Apart from the two small remarks it looks ok to me.
Thanks for expanding the documentation.

[JPBergsma]

Suggested change

	Example JSON config file fragment for adding two fields to each of the `structures` and `references` endpoints, that will be served as, e.g., `_exmpl_cell_volume` if the `provider.prefix` is configured.
	Example JSON config file fragment for adding two fields to each of the `structures` and `references` endpoints, that will be served as, e.g., `_exmpl_cell_volume` if the `provider.prefix` is set to 'exmpl'.

This seemed a bit clearer to me.

[ml-evs]

Thanks @JPBergsma, have addressed your comments and I'll merge myself without further review.