[apm][docs] Improve APM documentation structure and content

[bmorelli25]

As a prerequisite to the Observability documentation overhaul, we should begin to:

• Separate non-reference content from reference content -- Can we identify important tasks and create new task based guides? Topics like: Set up machine learning integrations, Create alerts, Configure APM Agents, Investigate slow requests, Investigate errors, etc.
• Improve TOC structure and headings -- We should move content out from the "Using APM" bucket and make use of table abbreviations to simplify the TOC headings.

Why?

Eventually, in the new Observability documentation, reference content will live separately from non-reference. We need a triage location for our guide content as we adapt what we have into how-to guides, and begin to create more task-focussed documentation.

New structure

A new structure for our current content could look like this. Additional work would still need to be done to adapt the content under the "Navigate the APM app section".

• Get started
• Navigate the APM app (content from Using APM & Visualizing Application Bottlenecks)
  -- Services overview
  -- Traces overview
  -- Transaction overview
  -- Span timeline
  -- Errors overview
  -- Metrics overview
  -- Filters
  -- Advanced queries
• How-to guides
  -- Configure APM Agents in Kibana
  -- Create a custom link
  -- Integrate with machine learning
  -- Query in the Discover app
• Settings
• API
  -- Agent Configuration API
• Troubleshoot

Related Issues

• Similar issue for APM Server: https://github.com/elastic/apm-server/issues/3458

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[bmorelli25]

Added a potential content layout, and additional context based on questions in elastic/apm-server#3492 (comment).