Change format of exported fields reference

[dedemorton]

As a follow up to #12242, we want to make additional changes to the layout of the exported fields docs (for example, https://www.elastic.co/guide/en/beats/metricbeat/current/exported-fields.html).

Phase 1:

• Remove sections that do not add value.
• Improve fields descriptions where needed.
• Replace each removed section with an entry that describes the group. For example, instead of having the hierarchy Apache fields > apache > status, we would remove the sections and instead have something that looks like this:

Phase 2:

• As soon our doc layout supports having columns that contain long field names, format the field descriptions in tables rather than definition lists. (Right now, the right-hand navigation severely restricts the allowed width of columns, and the asciidoc settings that control column width are not available with our current doc tool chain.)
• It might also be nice to revisit which info is important to users and how we present that info. For example, showing type: keyword takes up a lot of space. Could we present this info in a better way? Maybe in text in parentheses? In gray text? Another col in the table?

@ruflin FYI.

[dedemorton]

With future plans in mind, I don't think it makes sense to move forward with these changes, so I'm closing this issue.