Cleanup fields documentation

[simitt]

The current section for exported fields has some flaws:

• repetition of fields, see General APM fields fields
• unexpected structures, e.g. having context fields, but then also putting some of the context fields into other sections, https://www.elastic.co/guide/en/apm/server/current/exported-fields-apm.html
• including Kibana only fields

[simitt]

After some discussions we keep the fields as is for the moment.