Document valid data types and formatters in fields definitions

[jsoriano]

Fields defined in fields.yml files can define what is the datatype
for the ES indexes, and the formatter to be used by Kibana.
We should explain at some point of the devguide what are the
valid values, ideally with recommendations about which ones
to use on each case.

We are adding checks for known values in #13188.

[botelastic]

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[botelastic]

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