Adds RAG for alerts info to the AI Assistant page

[benironside]

Fixes #4456. Updates the AI Assistant page with information about the RAG for alerts feature that allows you to send multiple alerts as part of each prompt, so that AI Assistant can answer questions about recent alerts in your environment rather than just individual alerts.

Preview: AI Assistant

Related updates to the "Triage alerts with AI Assistant" page can be previewed here: #4359 . (RAG for Alerts changes are limited to the new first section "Use AI Assistant to Triage multiple alerts".)

[github-actions]

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[andrew-goldstein]

Thanks @benironside for your help with the Alerts setting!
Your feedback is included in elastic/kibana#173809

[andrew-goldstein]

Thanks @benironside for these updates to the KB section
✅ Desk tested via the preview link
LGTM 🚀

[joepeeples]

A few edits here, and two other things:

• The lead-in on line 147 says there are "four tabs" but now there are five. We should just omit numbering like this, which can easily get outdated and overlooked like this was, and just say something like "the following tabs."

• The section on the tabs has gotten quite long and jumbly, and I think we need a few subsections for the more in-depth config settings. We could have a simple overview bullet list of the five tabs, and then link to the subsections for more details.

[nastasha-solomon]

Left a handful of light edits. All around, lgtm!