Update Cake.Kudu.Client README: Add link to GitHub Discussions

We've started using [GitHub Discussions](https://github.com/cake-build/cake/discussions) as the preferred communication channel moving forward (instead of Gitter) because it makes it easier to keep track of discussions in a structured way, especially if multiple discussions are happening at the same time. It also allows to search for previous questions/answers, which can be a helpful resource.

As such, we're recommending addin maintainers to update any links to Gitter to point to the GH discussions.


Template:

```markdown
## Discussion

For questions and to discuss ideas & feature requests, use the [GitHub discussions on the Cake GitHub repository](https://github.com/cake-build/cake/discussions), under the [Extension Q&A](https://github.com/cake-build/cake/discussions/categories/extension-q-a) category.

[](https://github.com/cake-build/cake/discussions)
```

Open the repository README and find the existing Gitter communication link. Replace the relevant discussion guidance with the provided GitHub Discussions wording and links, then review the rendered README to confirm the new section is clear and no outdated Gitter link remains.

Written by the indexing model from the issue text.