Add the CADQuery-based sketch API

[lf94]

I've updated the documentation of docs/lib/Builder.rst and added docs/Engineering.rst.

cheatsheet.txt is not included yet. It will be in a future PR.

As mentioned a more cohesive reference needs to be written in the future, as the current documentation is not easy to use for most people. This reference will expand on how the engineering libraries can be used.

[doug-moen]

Thanks for the updates.
Don't forget about #143 -- I still consider lib.builder to be confusing and buggy until this is fixed.