[1.x] docs: Link to APM guide

[bmorelli25]

Backports #608 to 1.x.

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-01-20T17:09:24.301+0000

• Duration: 89 min 12 sec

• Commit: 7362d6d

Test stats 🧪

Test	Results
Failed	0
Passed	7596
Skipped	0
Total	7596

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)