Switch to AI Agent intro doc

[benironside]

Addresses #4204, and partly addresses #522 by creating a new page in the explore-analyze AI section that describes the new AI Agent chat experience, and informs users about the option to opt in to it in 9.3.

Hopefully this can be a starting point for the team to build on. I'm sure I didn't get everything right, and I didn't try to tackle the Observability or Search details, but I have a pretty good picture of the opt-in flow for Observability and Security, and the feature differences in the flyout chat experience for Security, so I wanted to get that on paper.

I also have this statement from @leemthompo bouncing around my head, reviewers take note:

What you [and Mike] need to figure out is what the diff is in the flyout experience versus the standalone chat experience. We can then determine if we can continue documenting everything under the AB umbrella, or if it makes sense to break out the flyout stuff into the Security + Obs solution docs, respectively.

I also decided not to document the pop-up opt-in option, but rather to just send people to the GenAI settings page if they want to opt in, since the opt-in popup experience won't always be available.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• [ x] No

[github-actions]

Vale Linting Results

Summary: 4 suggestions found

💡 Suggestions (4)

File	Line	Rule	Message
explore-analyze/ai-features/ai-chat-experiences/switch-to-ai-agent.md	20	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
explore-analyze/ai-features/ai-chat-experiences/switch-to-ai-agent.md	29	Elastic.Wordiness	Consider using 'some' instead of 'Some of the'.
explore-analyze/ai-features/ai-chat-experiences/switch-to-ai-agent.md	45	Elastic.Wordiness	Consider using 'also' instead of 'In addition'.
explore-analyze/ai-features/ai-chat-experiences/switch-to-ai-agent.md	45	Elastic.FutureTense	'won't show' might be in future tense. Write in the present tense to describe the state of the product as it is now.

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/ai-features/ai-chat-experiences/switch-to-ai-agent.md