Add latest Moco examples to the Doxygen examples list

[nickbianco]

These examples should be listed under both "API > Examples" and "API > Moco > Moco Examples" (which require updating doxyfile_shared.in and MocoExamples.dox, respectively):

• example2DWalking.py
• example2DWalkingMetabolics.py
• example3DWalking.cpp
• example3DWalking.py
• example3DWalking.m

[nickbianco]

Related, but maybe worth a separate issue. For some reason, in a recent build of the Doxygen documentation by @aymanhab for the 4.5.2 release, the CommonUtilities header file shows up under the list of examples: