Mention Gemini AI engine in introduction how-they-work docs

[arthurfvives]

The page AI Engines introduction does not currently mention Google's Gemini AI engine, even though the reference engines section does include 'gemini'.

Proposed update

• Update the introductory section at https://github.github.com/gh-aw/introduction/how-they-work/#ai-engines to mention "Gemini" as a supported engine, consistent with the reference documentation.

Context

• Reference documentation already lists gemini as a supported engine: https://github.github.com/gh-aw/reference/engines/
• Consistent documentation will help users quickly identify available AI engine integrations in both locations.

Plan

1. Update the introductory text in the "AI Engines" section to mention "Gemini" (by Google, as appropriate) as a supported engine.
2. Ensure the wording aligns with the Reference/Engines section.
3. Proofread for accuracy and consistency with style.