Run EA container + managed by fleet + standalone + autodiscovery

[constanca-m]

The main changes of this PR are:

• The elastic agent container
• Elastic agent managed by fleet
• Elastic agent standalone + autodiscovery

I tried to keep the common things like kube-state-metrics, deploy manifests etc in common docs files so it is easier to change them all at once if something changes in the future. I didn't change the common things in the new docs of managed by fleet.

[github-actions]

A documentation preview will be available soon:

• 📚 HTML diff
• 📘 Fleet and Elastic Agent Guide
• 📙 Observability Guide

[mergify]

This pull request does not have a backport label. Could you fix it @constanca-m? 🙏
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 /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflict. Could you fix it @constanca-m? 🙏
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 update-docs upstream/update-docs
git merge upstream/main
git push upstream update-docs

[tetianakravchenko]

In these cases, it won’t be possible to use scheduler metricsets, necessary for this example.

is scheduler is really necessary for this example? I mean in context of condition autodiscover?

It is quite confusing to have scheduler sample in between of redis examples.

[constanca-m]

I think it is important because readers get an idea how to set the host dynamically, and if we don't include it they may not get there on their own. I can only speak for myself, but as someone with little experience with Kubernetes, it would took me awhile to get there.

[tetianakravchenko]

but we have the similar configuration for the redis:

hosts:
        - '${kubernetes.pod.ip}:6379'

and with redis example can be achieved the same, no? (if there will be deployed multiple redis pods)

what I meant: it is confusing to get scheduler example, it doesn't seem to be relevant to what was said before - redis, and after - redis. Maybe it should be in separate block, not in == Example: Target Pods by label

[constanca-m]

At first it was indeed a separate block. But after discussion, it changed

[tetianakravchenko]

I still don't understand why example with scheduler is needed here, and what value does it bring if there exists already example with ${kubernetes.pod.ip}.

As you already have enough reviews - you can merge this PR

[dedemorton]

@constanca-m Will you be backporting this PR to 8.6? I tried to backport a change here, but noticed that your changes were not in 8.6. Let me know if you do decide to backport your PR so that I can make sure #2410 gets backported too. Thanks!