Search: clean up, clarify, and document all document fields

We have a lot of fields in the search index. Not all of them are necessarily useful and not all of them are the best way to use Elasticsearch. We should clean them all up, and then document them on: https://jetpack.com/support/search/advanced-search-document-fields/

A lot of these are documented [here](https://developer.wordpress.com/elasticsearch-post-doc-schema-v2/), but especially because of Gutenberg I am not certain that we should keep all of them. Also, fields that we decide not to document should probably get removed from the index.

Fields that exist but are not documented:
- the "deprecated" url fields
- location
- langs field (probability of each language)
- all of the content extraction stuff: https://developer.wordpress.com/elasticsearch-post-doc-schema-v2/#es-mapping-post-content-extracted

Start by comparing the existing Elasticsearch post document schema with the Advanced Search Document Fields page and the listed undocumented fields, including deprecated URL fields, location, langs, and content extraction. Determine which fields should remain indexed and which should be documented or removed. Done means the index fields and public documentation are aligned, with decisions covering Gutenberg-related fields.

Written by the indexing model from the issue text.