[Feature Request] Move async backing docs to `polkadot-sdk-docs`

[Kian Paimani]

as per discussed with @DrW3RK, since it is full of code snippets and it is easier to maintain it in polkadot-sdk, assuming there is one correct example of each snippet in that repo, which I believe it is.

[Radha]

Sure. PR in progress already https://github.com/DrW3RK/polkadot-sdk/pull/1