Generate documentation for imported external nested fields

[jsoriano]

While trying to remove duplicated fields to introduce package-spec 1.8.0 I found that sub-fields of external fields of type group were being included in the built fields files, but not in the documentation.

This PR attempts to address a couple of incoherences found with external fields, documentation and final generated field files:

• Fields imported as subfields of external fields are being included in the built package, but not in the documentation.
• Fields of type group, or that contain subfields are handled differently between defined and external fields:
  • Defined groups are not included in the docs, but their sub-fields are.
  • External groups are included in the docs, but their sub-fields aren't.

An alternative could be to don't import sub-fields of external groups, but this change could be breaking as it would modify the fields actually included in the mappings. The approach in this PR solves the incoherences by modifying only the docs, what shouldn't be breaking.

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• Start Time: 2022-05-09T17:28:27.798+0000

• Duration: 27 min 8 sec

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (30/30)	💚
Files	64.22% (70/109)	👍
Classes	58.278% (88/151)	👍
Methods	47.466% (281/592)	👍
Lines	32.011% (2574/8041)	👍
Conditionals	100.0% (0/0)	💚