doc: win: Add DTC and pip instructions

Add instructions to build the DTC from Windows, note that the following
2 caveats apply:

* The flex version needs to be pinned to 2.6.0 because of a bug with the
current MSYS2 flex

* The repository to clone DTC from is currently my own on GH while
waiting for a patch to be accepted upstream

Additionally this removes the python2 requirement and adds documentation
on installing pip an pyaml.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>