Add section about AI connector

[dedemorton]

Adds documentation requested in elastic/kibana#181282.

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This should be reviewed alongside elastic/kibana#183792 (the connector reference topic in the Kibana Guide).

TODO:

• Test info and add a screenshot.
• Add guidance around some of the questions asked and answered here.
• Clean up the description of the message sent to the AI Assistant.

TODO (after merging):

• Port to serverless. Add section about AI connector to serverless #3958
• Add link to connector docs after [DOCS] Obs AI Assistant connector kibana#183792 is merged (in this topic and to the list of valid connectors). Add link to published Obs AI Assistant docs #3957

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[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
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.

[mdbirnstiehl]

A couple of comments/questions, but otherwise LGTM!

[klacabane]

LGTM! Thanks for the change

[bmorelli25]

This is neat. 🐦

[dedemorton]

I'm going to merge this now to get it done then add the link after elastic/kibana#183792 is merged.