[docs] Add Markdown release notes

[colleenmcginnis]

🍷 Pairs with elastic/elastic-agent-changelog-tool#213 elastic/elastic-agent#9440

What is the problem this PR solves?

Currently the Elastic Agent Control Plane team generates release notes in AsciiDoc in this repo and the @elastic/ingest-docs team has to manually translate AsciiDoc to Markdown, copy to another repo (docs-content), split into multiple files, and integrate with Elastic Agent release notes. This is time consuming for the docs team and often leads to delays in publishing release notes.

How does this PR solve the problem?

In elastic/elastic-agent-changelog-tool#213, I updated the automation to allow generating Markdown, formatting it, and putting it into the correct files directly using elastic-agent-changelog-tool. This is the fleet-server output when generating the release notes using the colleenmcginnis:markdown branch of elastic-agent-changelog-tool.

This PR adds generated Markdown Fleet Server release notes here instead in the docs-content repo.

How to test this PR locally

This repo is not currently hooked up to the docs build system so we don't have a PR preview yet, but you can build the docs locally.

Note: Right now it will build independently from the rest of the Release notes, but once this repo is hooked up to the docs build system, these release notes will be integrated into the larger Release notes table of contents.

Design Checklist

This is a docs-only change.

Checklist

This is a docs-only change.

Related issues

• Update release notes for the Markdown era elastic-agent-changelog-tool#213
• [docs] Add Markdown release notes elastic-agent#9440
• https://github.com/elastic/ingest-dev/issues/5206
• https://github.com/elastic/docs-projects/issues/488

cc @ebeahan @pierrehilbert @bmorelli25

[prodsecmachine]

🎉 Snyk checks have passed. No issues have been found so far.

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[mergify]

This pull request does not have a backport label. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./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.

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
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