docs: added Callback to documentation and examples.

[maccesch]

Also reduced code duplication in Callback implementation.

[maccesch]

@gbj The failing CI is not from my code

[gbj]

Thanks, looks great!

[maccesch]

Oh I now have the Callback before the Fn() option. I figured that after the discussons in Discord you'd probably want me to reverse that order?

[gbj]

Don't worry about it, I can go in and rearrange them if I'd like!