Replace outdated information in README with link to full example registry.

[polina-c]

Contributes to #907

1. Replace outdated information in README with link to full example registry.
2. Fix capitalization of A2UI in documentation.

[gemini-code-assist]

Code Review

This pull request simplifies the examples/README.md by replacing the detailed example table with a link to the external a2ui example registry and updating the SDK name. Feedback suggests reverting the name change to maintain consistency with the rest of the repository and removing a trailing space on line 4.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[ditman]

No description provided.

Please add a description to the PR (and maybe a link to the issue it's solving, if there's one!)

[polina-c]

No description provided.

Please add a description to the PR (and maybe a link to the issue it's solving, if there's one!)

Done

[ditman]

Thanks for linking to the global examples directory!

Another file you can update is: https://github.com/flutter/genui/tree/main/packages/genui/example/README.md

That is currently linking to this general file, but you can save users 1 level of indirection by linking in the genui/example/README.md directly to the samples repo too.

(Note that genui/example/README.md is what's used by pub.dev, here: https://pub.dev/packages/genui/example)