remove outdated documentation link in README
remove outdated documentation link in README

The HCI URL redirects to the github.io one, but the README mentions both
as documenting different versions.

(Also, beautify the URLs by omitting /index.html.)

kod3r/vigra,funkey/vigra,xiwei-zhang/vigra,cha007/vigra,stuarteberg/vigra,funkey/vigra,stuarteberg/vigra,funkey/vigra,xiwei-zhang/vigra,mmuellner/vigra,dstoe/vigra,kod3r/vigra,funkey/vigra,dstoe/vigra,mmuellner/vigra,stuarteberg/vigra,timoMa/vigra,dagophil/vigra,dstoe/vigra,dagophil/vigra,cha007/vigra,ThomasWalter/vigra,hmeine/vigra,funkey/vigra,hmeine/vigra,kod3r/vigra,cha007/vigra,dstoe/vigra,kod3r/vigra,mmuellner/vigra,dagophil/vigra,ThomasWalter/vigra,cha007/vigra,timoMa/vigra,dagophil/vigra,timoMa/vigra,kod3r/vigra,mmuellner/vigra,hmeine/vigra,stuarteberg/vigra,xiwei-zhang/vigra,dstoe/vigra,capturePointer/vigra,funkey/vigra,dstoe/vigra,capturePointer/vigra,capturePointer/vigra,cha007/vigra,timoMa/vigra,cha007/vigra,timoMa/vigra,ThomasWalter/vigra,capturePointer/vigra,stuarteberg/vigra,mmuellner/vigra,xiwei-zhang/vigra,timoMa/vigra,xiwei-zhang/vigra,stuarteberg/vigra,kod3r/vigra,cha007/vigra,hmeine/vigra,xiwei-zhang/vigra,dstoe/vigra,hmeine/vigra,hmeine/vigra,mmuellner/vigra,capturePointer/vigra,kod3r/vigra,dagophil/vigra,xiwei-zhang/vigra,dagophil/vigra,ThomasWalter/vigra,ThomasWalter/vigra,stuarteberg/vigra,dagophil/vigra,ThomasWalter/vigra,capturePointer/vigra,ThomasWalter/vigra,mmuellner/vigra,hmeine/vigra,timoMa/vigra,capturePointer/vigra,funkey/vigra