doc/conf: Reintroduce sys.path setting

e67cb1b3 ("Merge pull request #658
from douglas-raillard-arm/lisa-next") broke ReadTheDocs, and while it
looked like it was due to some misuse of sys.path (which was removed
in ac276593 ("doc/conf: Remove path setting")), it looks like we
do need to play with sys.path to keep ReadTheDocs up and running.