Improve Custom Logs documentation

[rdner]

What does this PR do?

Now it describes detailed steps how to set it up.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
  - [ ] I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

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Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

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• Start Time: 2023-01-31T16:40:29.896+0000

• Duration: 13 min 23 sec

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Test	Results
Failed	0
Passed	1
Skipped	0
Total	1

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (0/0)	💚
Files	100.0% (0/0)	💚
Classes	100.0% (0/0)	💚
Methods	33.333% (1/3)	👎 -54.74
Lines	100.0% (0/0)	💚 8.02
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Package log - 1.1.1 containing this change is available at https://epr.elastic.co/search?package=log