make it easier to get from State to Extension

[lolbinarycat]

Router::with_state mentions Extension as an alternative to State, but only after scrolling past 4 examples. extract::State doesn't mention Extension at all.

extensions are more modular, as they can be added to various handlers while keeping other handlers pure.

[Lachstec]

Hi there! If I understand you correctly, you think it would be nice to mention the concept of Extension way earlier in the docs so that people reading it are made aware of their existence, as they can be a more suitable solution to their problem as relying on State?

I also think that the docs could be improved in that regard and would take a shot at it.

[lolbinarycat]

yep, that is correct!

[Lachstec]

I openend a PR (#2991) , would love your Feedback. It's just what first came to my mind and can surely be improved further.