docs: add preoptimizing your images documentation

[oorestisime]

Contributes: #11798

I am not sure this was the best place possible to add this part of documentation.
Let me know if you think it is more appropriate to move this elsewhere (in plugin-sharp ?)

[oorestisime]

@DSchau i wasn't sure how to specify that you did the script. i was thinking to add it as a comment in the code example.

[oorestisime]

I need to update this based on the discussion on twitter yesterday. Didn't yet have the time to do it.

[DSchau]

Tweaked a few things, but merging this in.

Thanks for this--it's very much appreciated!