[8.14] AI Assistant image updates, edits from twin PR, model recommendation (backport #5237)

[mergify]

Fixes #5040. This is a twin PR of #344, except two differences:

• The connector setup guides were already present in ESS docs, so weren't added here.
• This PR updates the "Recommended models" section - so this is the only text in need of a review. The rest was already reviewed in the twin PR.

Preview: AI Assistant

---

This is an automatic backport of pull request #5237 done by Mergify.

[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.}