[docs] Documentation restructure

[bmorelli25]

WIP. For #1427.

This PR accomplishes the following:

• Use shared version attributes
• Rename the APM UI to "APM app"
• Updates to the new documentation structure
• Adds a top level heading for log correlation
• Adds a section on how the Agent does instrumentation
• Closes [docs] Fix duplication on getting started pages and api calls #1023 (duplication in getting started pages).

New layout (only one link changes)

[Qard]

Just left some quick suggestions for the intro doc, since you asked for it. I haven't done a full review. Let me know when you want one.

[bmorelli25]

Fantastic, thank you! Still some work to do, so I probably won't open this up for a full review until tomorrow. I'll let you know!

[bmorelli25]

@Qard this is ready for a final review

[bmorelli25]

It looks like it doesn't like the commit naming from the GitHub suggestions: Update docs/intro.asciidoc. Do I need to rename those?

[Qard]

Yes, all commits need to follow the type(subsystem): message or type: message forms. I'd just squash the commits, but we can also do that when landing, assuming everything else passes.

[bmorelli25]

Finally got this green @Qard

[Qard]

LGTM other than one formatting question. Also, I'd like to backport this to 2.x so we can minimize the delta between versions allowing future doc fixes to be backported easily too. Any issues you foresee with that?

[bmorelli25]

I agree with backporting these changes. I expect a few collisions, but don't see any major problems getting this backported.