Present `gSystem.CompileMacro()` as way to load c++ stuff from Python

[hageboeck]

The PyROOT section lists several ways to access C++ stuff from Python. What's missing is the easy-to use

ROOT.gSystem.CompileMacro("ClassImplementation.cpp", "kO")

It's important to use kO for performance or if it happens frequently, so those should be explained.

Further, if this section is touched anyway, one could revisit the preferred way of loading c++ from Python. There should preferably only be a few ways that will always do what the users want.