Depends system needs to figure more prominently in build docs #7473

Some people have spent a lot of time figuring out how to build dependencies, or even how to cross-compile them, before realizing that the depends system makes this trivial. This is because the documentation is a bit hidden, it is not linked from build-unix.md for example.

Also needs an example of cross-compiling, see here :https://github.com/bitcoin/bitcoin/issues/7471#issuecomment-180261245

Looks like we already do that for Windows https://github.com/bitcoin/bitcoin/blob/master/doc/build-windows.md Forgot about that.