Add coordinate system transformations page

- Possible wording: "It's tricky to explain, but the point is that the type system means you shouldn't _need_ to think too carefully about it"
- Discuss chicken and egg problem: even if most code out in the wild right now just largely sticks to one global coordinate system, that's quite possibly because tracking/converting between different coordinate systems is confusing and error prone
- "`elm-geometry` does one thing that, to my knowledge, no other geometry library does: it tracks units and coordinate systems at compile time to avoid unit conversion and local vs global coordinate errors. While this may initially seem complex or burdensome, I have strived to make it as frictionless as possible in practice: in many cases only type annotations are affected, while implementation code is exactly as it would be without any compile-time units/coordinates checking. And I think this opens up a lot of possibilities to be free to use different units and coordinate systems in different parts of your code, without having to worry about accidentally mixing them up!"