Printed parts in documentation should link to STL files

Right now the documentation is still really unclear, as the links to printed parts don't actually ever go to the STL files.

Maybe we should reconsider a kind of STL pack generator on the website? The Git Building part pages are fine but you have to jump through a load of hoops just to get the right STL files for your build.

Any thoughts @rwb27 @julianstirling ?