Update learning resources index to match glossary styling

[ericwindmill]

Page URL

https://docs.flutter.dev/reference/learning-resources

Page source

https://github.com/flutter/website/blob/main/site/lib/src/components/pages/learning_resource_index.dart

Describe the problem

The learning resource index and glossary page (and maybe other pages) have different styles applied to the search bar and the cards.

Glossary page:

Learning resource index

Expected fix

The learning resource index styles match the styles of the glossary page. Ideally, it reuses the same code as much as possible

Additional context

No response

I would like to fix this problem.

• [ ] I will try and fix this problem on docs.flutter.dev.