[sidebar] Consider moving `Flutter concepts` to `Learn Flutter` for clearer organization and visibility

[guidezpl]

Page URL

https://docs.flutter.dev/resources/

Page source

https://github.com/flutter/website/tree/main/src/content/resources/architectural-overview.md

Describe the problem

If approved good, can do this

Expected fix

No response

Additional context

No response

I would like to fix this problem.

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