DOC: Update Documentation/Introduction.md for GitHub migration
DOC: Update Documentation/Introduction.md for GitHub migration

Also fix paths to referenced documents.

BRAINSia/ITK,spinicist/ITK,fbudin69500/ITK,malaterre/ITK,thewtex/ITK,malaterre/ITK,spinicist/ITK,vfonov/ITK,malaterre/ITK,thewtex/ITK,blowekamp/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,hjmjohnson/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,spinicist/ITK,thewtex/ITK,blowekamp/ITK,vfonov/ITK,LucasGandel/ITK,Kitware/ITK,fbudin69500/ITK,LucasGandel/ITK,blowekamp/ITK,fbudin69500/ITK,Kitware/ITK,malaterre/ITK,hjmjohnson/ITK,thewtex/ITK,vfonov/ITK,thewtex/ITK,spinicist/ITK,vfonov/ITK,malaterre/ITK,LucasGandel/ITK,vfonov/ITK,spinicist/ITK,hjmjohnson/ITK,richardbeare/ITK,richardbeare/ITK,malaterre/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,vfonov/ITK,fbudin69500/ITK,vfonov/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,spinicist/ITK,Kitware/ITK,Kitware/ITK,LucasGandel/ITK,fbudin69500/ITK,malaterre/ITK,LucasGandel/ITK,vfonov/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,fbudin69500/ITK,thewtex/ITK,fbudin69500/ITK,thewtex/ITK,BRAINSia/ITK,hjmjohnson/ITK,fbudin69500/ITK,malaterre/ITK,BRAINSia/ITK,malaterre/ITK,richardbeare/ITK,hjmjohnson/ITK,BRAINSia/ITK,BRAINSia/ITK,spinicist/ITK,hjmjohnson/ITK,spinicist/ITK,spinicist/ITK,blowekamp/ITK,Kitware/ITK,richardbeare/ITK,blowekamp/ITK,Kitware/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,richardbeare/ITK