Add docs for Kafka output UI for Fleet managed agents

[kilfoyle]

This adds docs for the new Kafka output settings UI

• Setting descriptions came mainly from the requirements definitions doc and the Filebeat Kafka output docs.
• Kafka output is marked as in technical preview.
• I split the Elasticsearch, Logstash, and Kafka output settings sections into separate sub-pages.
  

Preview site

Closes: #68

[mergify]

This pull request does not have a backport label. Could you fix it @kilfoyle? 🙏
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 @kilfoyle? 🙏
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 <remote-repo>
git checkout -b 68/kafka-output <remote-repo>/68/kafka-output
git merge <remote-repo>/main
git push <remote-repo> 68/kafka-output

[kilfoyle]

Removed the Channel Buffer Size parameter per elastic/kibana#162875 (comment)

[criamico]

Hi @kilfoyle, I think it would be good to involve @szwarckonrad in this review as he actually worked on implementing the feature so he has more in-depth knowledge of the kafka options. I'll request his review as well.

[kilfoyle]

@faec Would you mind taking a look at these descriptions of the Kafka authentication settings, just to be sure I'm on the right track?

[juliaElastic]

LGTM overall, haven't checked the Kafka settings in detail.
It would be nice to add some screenshots about where to find the Add output button in Fleet settings, and how the Add new output flyout looks like. Though the instructions are there as text, so that's fine too.

[szwarckonrad]

Looks great! I've added a few comments - just some suggestions on how we might enhance clarity.

Another consideration arose today - whenever an user selects the Kafka output type, we will display the warning detailed in this ticket and depicted in this pull request.

[kilfoyle]

Thanks for the excellent feedback!

@szwarckonrad I adopted all your suggestions (via f0e68b3) and added a tip at the top of the docs page Kafka output is currently not supported on {agents} using the {elastic-defend} integration..

@juliaElastic Good idea to add a graphic. I've added one to help people find the Add output button. I didn't add the Add new output UI itself. We try to minimize showing larger UIs since they change frequently and the docs can go out of sync easily.