Documentation tweaks

[benhutchison]

This is some feedback on two small ways Coulomb docs could be slightly more beginner friendly.

It's no exaggeration to say that Coulomb has a lengthy learning curve. There are a lot of details to learn and remember to use the library fluently and productively. I have 15 years with Scala and am already convinced of the value proposition, and yet it's been challenging to adopt Coulomb practically in a "real world" codebase.

1. Revise key imports on the Concepts page.

   The Introduction page includes a section on imports. However, for the first year of use, a programmer will be spending a lot of time on the Concepts page which contains a lot of key documentation. I find myself often flicking back to the intro just to grab the imports again. So I suggest a small section that revises the assumed imports at the top of Concepts.

2. Mention Unitless quantities. Scalars play a very important role in most domains. When embracing units, it's important to know how to "turn them off". A paragraph with an explanation of unitless quantities and how to simplify canceling units back to unitless would be useful. Eg The use of literal type 1 for scalars might be surprising to newcomers.

[erikerlandson]

@benhutchison good point, I never included a discussion of unitless values, which I had planned to do