Cherry-pick #24588 to 7.12: Add docs about PostgreSQL monitoring

[jsoriano]

Cherry-pick of PR #24588 to 7.12 branch. Original message:

What does this PR do?

Add documentation in PostgreSQL modules about recommended configuration.

Why is it important?

To better leverage PostgreSQL modules.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes [Filebeat] Improve documentation for Filebeat PostgreSQL module #14337.
• Related to [Filebeat PostgreSQL] Query Duration Overview dashboard has spurious query #20321.
• Part of Migrate postgresql integration integrations#366.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

Pinging @elastic/obs-docs (Team:Docs)

[elasticmachine]

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Build stats

• Build Cause: Pull request #24745 updated

• Start Time: 2021-03-29T17:15:29.389+0000

• Duration: 51 min 28 sec

• Commit: df8c828

Test stats 🧪

Test	Results
Failed	0
Passed	21694
Skipped	4355
Total	26049

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Test stats 🧪

Test	Results
Failed	0
Passed	21694
Skipped	4355
Total	26049