Doc: Restructure content for field references

[karenzone]

The Field References topic is too low in the content architecture. When we enhanced that section, we ran out of heading levels and created the "Deep Dive" topic as a work around.

ToDo: Move the Field References topic higher in the doc architecture and pull in the content that's currently in the "Deep Dive" topic .

Related:

• https://www.elastic.co/guide/en/logstash/current/configuration-file-structure.html#field-reference
• https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#logstash-config-field-references⭐
  • https://www.elastic.co/guide/en/logstash/current/field-references-deepdive.html