Add AG-UI adk-middleware tutorial

[maxkorp]

This adds a tutorial page for using the AG-UI adk-middleware to enable a python ADK agent to use AG-UI.

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[zeroasterisk]

The header needs a little more use case, and the distinction between AG UI and CopilotKit needs work, and a bit of positioning. I think we can make it work.

Also, this seems pretty long to me, stylistically... But perhaps not. I'll defer to tech writers review

[koverholt]

Thanks for your contribution, the ADK + AG-UI work is very exciting stuff!

In its current form, this PR duplicates docs content from the following existing AG-UI docs pages:

• https://docs.copilotkit.ai/adk/quickstart
• https://docs.copilotkit.ai/adk/agentic-chat-ui
• https://docs.copilotkit.ai/adk/generative-ui/agentic
• https://docs.copilotkit.ai/adk/generative-ui/tool-based
• https://docs.copilotkit.ai/adk/human-in-the-loop/agent
• https://docs.copilotkit.ai/adk/shared-state/in-app-agent-read
• https://docs.copilotkit.ai/adk/shared-state/in-app-agent-write

and puts them into a single page in the ADK docs, which seems suboptimal from a scope and maintenance perspective. When presenting AG-UI from the perspective of the ADK docs, we want users to learn about the integration, see a quick prototype, and then dive into the full AG-UI docs for more comprehensive tutorials and components docs.

Some alternative options that would make this fit better in the ADK docs:

• Create a new simplified, shortened tutorial that covers agentic UI and tool use with simpler ADK code (but does not cover other cases like HITL or shared state), and instead link to the AG-UI docs for those workflows
• Rework this page to be more of an overview of ADK + AG-UI functionality with Overview, Quickstart, and Resources sections (similar to a page such as https://google.github.io/adk-docs/observability/arize-ax/)
• (Longer term future) Create docs pages with guidance on developing frontends with ADK, and include an overview page with AG-UI as one option for doing so.

cc @joefernandez for his suggestions and guidance on the direction of this AG-UI content, since we have been discussing the scope of tutorials and integrations such as this.

[maxkorp]

@zeroasterisk and @koverholt I think I got everything addressed, let me know what you think!

[koverholt]

Thanks for the edits, this is looking great! I made some additional tweaks to add the page to the nav and other formatting edits.

You'll need to sign a Contributor License Agreement (CLA) before we can merge this, and you can find more info in the CLA check output: https://github.com/google/adk-docs/pull/743/checks