docs: Update monitoring

[bmorelli25]

Summary

This PR combines our monitoring docs into one cohesive section. See the preview to feel it out.

[mergify]

This pull request does not have a backport label. Could you fix it @bmorelli25? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[ghost]

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• Start Time: 2023-05-16T21:47:51.805+0000

• Duration: 2 min 21 sec

🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate and publish the docker images.

• /test windows : Build & tests on Windows.

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

[simitt]

. Start {agent}
+
Use the command that work with your system:

The third step in this guide is actually not true anymore. Users are required to re-enroll the elastic agent after changing the elastic-agent.yml, otherwise the changes won't have an effect.

[mergify]

This pull request is now in conflicts. Could you fix it @bmorelli25? 🙏
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 ft-monitor upstream/ft-monitor
git merge upstream/main
git push upstream ft-monitor