docs: Add additional APM integration information

[bmorelli25]

Addresses most of the to-dos in the APM integration docs for 7.12. For now, this information is very basic.

For #4494.

[codecov-io]

Codecov Report

Merging #4969 (c732f63) into master (c077005) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4969   +/-   ##
=======================================
  Coverage   76.98%   76.98%           
=======================================
  Files         175      175           
  Lines       10354    10354           
=======================================
  Hits         7971     7971           
  Misses       2383     2383           

Impacted Files	Coverage Δ
...ack/apm-server/aggregation/txmetrics/aggregator.go	93.24% <0.00%> (ø)

[apmmachine]

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Build stats

• Build Cause: Pull request #4969 updated

• Start Time: 2021-03-18T00:25:00.673+0000

• Duration: 45 min 50 sec

• Commit: 87e6d77

Test stats 🧪

Test	Results
Failed	0
Passed	4785
Skipped	117
Total	4902

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