[8.16](backport #4326) AI Assistant docs updates for 8.16

[mergify]

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)

---

This is an automatic backport of pull request #4326 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 20cc3dc has failed:

On branch mergify/bp/8.16/pr-4326
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit 20cc3dc7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	deleted:    docs/en/observability/images/ai-assistant-button.png
	new file:   docs/en/observability/images/icons/ai-assistant-bw.svg
	new file:   docs/en/observability/images/icons/ai-assistant.svg

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/observability/observability-ai-assistant.asciidoc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[eedugon]

@alaudazzi , @bmorelli25 : I realized that I had forgotten to add backport-8.16 label to the original PR (not sure if at that time it was needed), and I have added it now.
That has created this branch which apparently has a conflict... would you let me know how to proceed? Is this PR needed?

Thanks!