[8.x](backport #4143) O11y AI Assistant - Google Gemini support

[mergify]

We document Google Gemini in the connectors https://www.elastic.co/guide/en/kibana/current/gemini-action-type.html and in our release highlights https://www.elastic.co/guide/en/observability/current/whats-new.html, but it's missing from the o11y docs.

Add this mention to any place we list the supported connectors with the O11y AI Assistant.

TODO after merging:

• Port to serverless

  ---

  This is an automatic backport of pull request O11y AI Assistant - Google Gemini support #4143 done by Mergify.

[mergify]

Cherry-pick of 4143855 has failed:

On branch mergify/bp/8.x/pr-4143
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	both modified:   docs/en/observability/observability-ai-assistant.asciidoc

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[dedemorton]

LGTM (assuming CI passes)