update docs for reference<D, U> and remove operator/(Rep, reference)

[JohelEGP]

I also found warnings here:
https://mpusz.github.io/units/reference/systems/isq/dimensions_and_concepts.html
https://mpusz.github.io/units/reference/random.html
https://mpusz.github.io/units/reference/core/types/prefixes.html

[JohelEGP]

With this, reference<D, U> is pretty much what was suggested at #48 (comment). Can't underestimate their opinion.

[mpusz]

Thanks!