doc: gsg: macOS: Include instructions to add homebrew to the path

Multiple users have failed to read the output from the Homebrew
installation script, which instructs the user how to add it to the
path. Include the instructions in the guide.

Also add a step to include the Homebrew Python executable to the path,
which allows then for invocation of python and pip as well as python3
and pip3.

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