Windows: Add support to compile python api docs from make file

This adds support to compile the html python api docs from the command line by running `make doc_py` matching support between windows and unix.

This patch also makes it so the compiler is not needed if you set the `blender_bin` variable, this affects icon generation as well.

In the future, I want to move away from generating the build output in the build directory but that can come in a later change.

Reviewed By: LazyDodo

Differential Revision: https://developer.blender.org/D12144

build_files/windows/show_help.cmd

@@ -31,6 +31,10 @@ echo - 2019 ^(build with visual studio 2019^)
 echo - 2019pre ^(build with visual studio 2019 pre-release^)
 echo - 2019b ^(build with visual studio 2019 Build Tools^)
 
+echo.
+echo Documentation Targets ^(Not associated with building^)
+echo - doc_py ^(Generate sphinx python api docs^)
+
 echo.
 echo Experimental options
 echo - with_opengl_tests ^(enable both the render and draw opengl test suites^)