docs: update feedback and design.md links in READMEs

[victoreronmosele]

Description

This PR updates the README feedback links to the A2UI repo and updates the design.md link to its new location.

Fixes: a2ui-project/a2ui#1888

Related: #953

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog. N/A
• I updated/added relevant documentation (doc comments with ///). N/A
• If my PR is a fork PR, I've checked that [e2e tests] passed. N/A

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[gemini-code-assist]

Code Review

This pull request updates repository and issue tracker links in the README files to point to the new 'a2ui-project/a2ui' repository, and corrects the relative path to the design document. The feedback suggests improving the readability and accessibility of the design document link by using more descriptive text instead of the literal filename.

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

[gemini-code-assist]

Using the literal filename 'design.md' as the link text is less descriptive. It is better to use more natural, descriptive text for links to improve readability and accessibility.

Suggested change

	See [design.md](../../docs/contributing/design.md) for more detailed information about the design.
	See the [design document](../../docs/contributing/design.md) for more detailed information.

[victoreronmosele]

It was previously DESIGN.md and I just changed it to match the new file name.

I'll leave this decision to the maintainers.