[documentation][exported-fields] update autogeneration scripts to generate markdown

[VihasMakwana]

Proposed commit message

This is a follow-up of #44638.
This PR takes care of autogeneration of exported-fields.md document for each beat.

Unfortunately, it is not possible to break down this into PR for each beat because this particular autogeneration script is shared by each beat and is under libbeat. Updating dev-tools/mage/docs.go and libbeat/scripts/generate_fields_docs.py will update generate exported-fields for each beat.

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.

How to test this PR

• Head to docs-preview and verify the docs the Exported Fields section for each beat.

Related issues

• Relates Update Beats code to support new Markdown documentation #43008

[github-actions]

🤖 GitHub comments

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Just comment with:

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @VihasMakwana? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

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

[VihasMakwana]

Adding @AndersonQ and @mauri870 as reviewers, as they reviewed the first PR.

[colleenmcginnis]

I browsed through the Markdown files. I took a couple notes on some formatting we might be able to improve in the descriptions in the source YAML files later, but there's nothing blocking. Thanks again for your work on this!

[AndersonQ]

I checked a few exported fields page for every beat, they're there so it looks good!