docs: Sync Libbeat documentation

[bmorelli25]

Summary

Originally a follow-up to #5113, but turned into something more. This PR copies over documentation from the Beats repo.

I think everything in this PR makes sense in the APM Server Reference, but I'd appreciate someone from the apm-server team taking a look.

More information

This PR backports documentation changes from the following PRs:

• kafka.asciidoc 7.12 — [libbeat] Expose the new Sarama flag 'DisablePAFXFAST' in the Kafka output beats#23629 & Add "mechanism" in output.kafka to support SCRAM-SHA-512 and SCRAM-SHA-256  beats#12867
• logging-config.asciidoc 7.12 — Add syntax for multiple selector logging beats#24207
• shared-ssl-config.asciidoc — 7.12 (restructure) [DOCS] Restructure content for SSL settings beats#24342
• shared-path-config.asciidoc 7.13 — Refactor use of system.hostfs to fix cgroup metrics beats#24334
• fileout.asciidoc — master [Enhancement] Add RotateOnStartup feature flag for file output beats#19347

Backport to...

Commits in this PR need to be backported to different branches:

• 7.x, 7.13, 7.12 — d77347f & 13b5820 Too many conflicts. Cancelling the backport to 7.12.
• 7.x, 7.13 — a7a7d66 & de79e36
• Do not backport — c7f054f

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Build stats

• Build Cause: Pull request #5185 updated

• Start Time: 2021-04-29T21:32:17.772+0000

• Duration: 38 min 4 sec

• Commit: de79e36

Test stats 🧪

Test	Results
Failed	0
Passed	6294
Skipped	120
Total	6414

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[axw]

Spotted one small thing, which I guess will need fixing in the beats repo. LGTM otherwise