Save documentation in ./docs directory

[alexgig]

./elm-docs is a bit redundant and unclear.

[cscalfani]

I know we've talked about this on Slack, but I want to put this here as a reminder when we revisit this.

I like that elm-docs sticks out as an internal directory like node_modules and elm-stuff. But more important than that is the fact that it won't accidentally collide with repository directories.