[8.13](backport #3663) Update observability AI assistant docs with latest changes

[mergify]

Adds changes to stateful for #3652. Note that we decided not to document "internal" functions.

Also alphabetizes the table of functions.

Preview link: https://observability-docs_bk_3663.docs-preview.app.elstc.co/guide/en/observability/master/obs-ai-assistant.html

Question: Why don't we alphabetize the list of functions in the UI? I know we sometimes like to put the most commonly used options first, but in my experience that kind of organizational approach falls apart over time.

Open questions (these questions also appear inline in the diff):

• What is the best link to use for AWS Bedrock security? The one I’ve provided in the updated docs, or the link that’s provided in the AI Assistant? RESOLVED: Leaving the link as-is.
• Should we replace the table of functions in the docs with the UI instructions for showing the list of functions (like we did in serverless)? There are some good arguments for doing this, but I wonder if seeing this info might actually be useful for people who are exploring the docs to figure out what is possible. If so, it’s better to keep the table, IMO. In not, then maybe just telling users how to see the list in the UI is good enough. RESOLVED: Removing the docs for internal functions and leaving the other functions documented.
• Has the "recall" function really has been renamed to "context". Can we just remove the description or do we need to mention that it’s been renamed? RESOVLED: Removing the docs for these functions.
• Is the list of functions documented in this PR correct for 8.13? Only a subset are actually shown in the UI. None of the apm functions are shown in the UI right now (maybe because I don't have APM set up?):
  
  If I ask the AI Assistant what functions are available, it'll give me a long list, including context and query, but that list does not show up in the UI when I click Select a function. RESOLVED: We will remove the internal functions.
• Is the description of the query function in this PR true? (Or is AI making stuff up?) RESOVLED: Yes, but we will remove the query function because it's an LLM-only function.

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This is an automatic backport of pull request #3663 done by [Mergify](https://mergify.com).

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