Added note about imports where necessary

[Tamnac]

Closes #12936

I tried my best to figure out the appropriate thing to import, but it turned out more complicated than expected, so may not be perfect.

Uses a NOTE in the overview section of each page.

[Tamnac]

I believe that's it. I must've forgotten to check YAML.

[Sija]

I'd rather use term require, since import is sth non-existent in Crystal, so far and it could be easily mistaken with import features in other languages, which are quite different.

[Tamnac]

I avoided repeating "require" since that's awkward phrasing, and "include" is already a keyword. I can make another pr though if you say so, since it seems this one was merged by mistake.

[HertzDevil]

The terminology is appropriate though, since you only require a file, not a type.

[straight-shoota]

Thanks for raising this issue. But I think it's okay.
The exact term is not critical.

[Sija]

The terminology is appropriate though, since you only require a file, not a type.

@HertzDevil Yeah, but the word in question is import.