Add section about AI connector

[dedemorton]

Adds documentation requested in https://github.com/elastic/kibana/issues/181282.

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This should be reviewed alongside https://github.com/elastic/kibana/pull/183792 (the connector reference topic in the Kibana Guide).

TODO:

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

TODO (after merging):

• [x] Port to serverless. https://github.com/elastic/observability-docs/pull/3958
• [x] Add link to connector docs after https://github.com/elastic/kibana/pull/183792 is merged (in this topic and to the list of valid connectors). https://github.com/elastic/observability-docs/pull/3957

[github-actions[bot]]

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[mergify[bot]]

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.

[dedemorton]

I'm going to merge this now to get it done then add the link after https://github.com/elastic/kibana/pull/183792 is merged.