Add docs for the Standard library: `std/`

[rtulip]

Now that a lot more is included by the prelude, none of the tutorials require an include statement any more. We need to maintain some good documentation about what's in the standard library.

[wfzyx]

hey I can work on this, do we have a standard/prefered way to add the docs on the functions at std/ or just adding a comment on top of the function declaration is fine?

[rtulip]

For now comments above the function declaration would be great.