docs: update release docs for v9

[endorama]

Motivation/summary

Update release docs to account for the new doc structure. 9x and 8x releases are mostly similar so far, but they may diverge in the future. To account for this I split them in 2 separate files, so the execution flow is linear and does not have conditional branches based on version.

Checklist

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Review the docs (or follow them on the next release ;))

Related issues

https://github.com/elastic/apm-server/issues/16181

[mergify[bot]]

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

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.
• 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.

[endorama]

Not backporting as we leverage main docs for all releases.

[endorama]

I forgot to set this to draft, the 9.x docs is not yet fully ready.

[mergify[bot]]

This pull request is now in conflicts. Could you fix it @endorama? 🙏 To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b release-docs upstream/release-docs
git merge upstream/main
git push upstream release-docs

[endorama]

Closed in favour of #17416