AI Assistant docs updates for 8.16

[eedugon]

Description

AI Assistant doc updates for 8.16, applying changes requested in #4273.

Preview link: https://observability-docs_bk_4326.docs-preview.app.elstc.co/guide/en/observability/master/obs-ai-assistant.html

The PR includes:

• Added Enterprise Search requirement in case Search Connectors are used.
• Included Search Connectors as a way to add data to the AI Assistant KB (Use Search Connectors section)
• Updated names for the other methods (knowledge base UI and Index API)
• Updated the icon related screenshot.
• Added AI Assistant Settings section with screenshot.

Pending:

• Determine if we should update the rest of the screenshots and gif --> This will be handled in a separate issue / separate PR, and we will avoid using screenshots if not needed (thanks @bmorelli25 ).
• Determine if we can remove the method about using the Index and Reindex API calls to add data to the AI Assistant KB, as it's too advanced and also doesn't appear completely accurate (maybe we could move that to another type of document) --> Moved to a new issue ([Request]: Update the reindex method on the Observability AI Assistant doc #4352) for a later stage and decision.
• Should we add a clarification about the different nature of Search Connectors (used to populate the KB) and Stack Connectors (used to connect to LLM). Users could mix them up. --> Agreed to add a TIP in the doc.
• I still need to add some details about Search Connectors but I was unable because of technical issues in the lab environment.
• Backporting checks: Are the changes in this PR valid for other releases? If Search Connectors were also integrated with the AI Assistant in other releases I'd say yes, but I'm not sure about it.
• Port to serveless decision (depends on the final content of this PR) --> Decided to not port to serverless as Search Conectors cannot be used there to populate the AI Assistant knowledge base.

Other concerns:

• Search Connectors wording: Should we use Connectors or Search Connectors? I think Connectors is more ambiguous considering we have other type of connectors.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #4273

Checklist

• Product/Engineering Review
• Writer Review

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

Serverless docs not updated due to #4273 (comment)

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[mergify]

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

[eedugon]

I have added the Search Connectors related content after performing some tests in a 8.16-SNAPSHOT lab, and after reading https://www.elastic.co/observability-labs/blog/elastic-rag-ai-assistant-application-issues-llm-github

The content is aligned with the shared blog post.

[alaudazzi]

LGTM