[docs] APM Overview structure and content improvements

[bmorelli25]

Preview this PR on Firebase: https://elastic-obs-docs.firebaseapp.com/index.html
😮 🆒 😎 🎉

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Backports

• 7.x - [docs] APM Overview structure and content improvements (#2171) #2424
• 7.3 - [docs] APM Overview structure and content improvements (#2171) #2423
• 7.2 - [docs] APM Overview structure and content (#2171) #2422
• 7.1 - [docs] APM Overview structure and content improvements (#2171) #2425
• 7.0 - [docs] APM Overview structure and content improvements (#2171) #2426
• 6.8 - [docs] APM Overview structure and content improvements (#2171) #2428
• 6.7 - [docs] APM Overview structure and content improvements (#2171) #2427

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This PR improves the content and structure of the APM Overview:

• Shifts to a cloud-first approach for installation
• Improves the layout of tables
• Refreshes old images and screenshots
• Adds new architecture images - closes [docs] Update APM Architecture diagram to include cloud #2117
• Adds a new "Troubleshooting" top level heading
• Adds a new "Real User Monitoring" top level heading
• Updates the overview
• Updates the components and documentation links

Proposed new structure:

• Overview (What is APM?)
• Components and documentation (What are the components and where can I find the relevant documentation?)
• Install and run (Cloud first)
  • Step 1: Install Elasticsearch
  • Step 2: Install Kibana
  • Step 3: Install APM Server
  • Step 4: Install APM agents
• Data Model (What kind of data does APM capture?)
  • Spans
  • Transactions
  • Errors
  • Metrics
  • Metadata
• Distributed tracing
• Real User Monitoring (RUM)
• OpenTracing bridge
• Agent and Server compatibility
• Troubleshooting
• Breaking changes
• Release highlights

Other notes:

• These changes are similar to the changes proposed for APM Server in (WIP) [docs] APM Server install and run structure #2170.
• I left out a "getting started" section until we're able to define exactly what that is.

[bmorelli25]

The failing elasticsearch-ci/docs-build is unrelated (it's really a bug).

[bmorelli25]

cc @tbragin

[tbragin]

@bmorelli25 Love this change! I see it in master:
https://www.elastic.co/guide/en/apm/get-started/master/install-and-run.html#install-and-run

How far back do you think we can backport it?

[tbragin]

Small nit -- since the page above is so short, I don't know if we need the "Alternatively, you can install the stack yourself." since the install the stack option is visible above the fold.

[bmorelli25]

Thanks Tanya. Good point with the nit, I'll get that fixed.

Backporting to 7.0 will be easy. Everything before that will take a little bit of work. I'm going to try and take this as far as I can. Probably 6.6 or 6.5.