ISIS Docs build fix for incorrect path and -WEBHELP issues (#4511)

* Add missing dot command from graphviz package

When building ISIS documentation, the dot command is not found. This dependancy is in the graphiviz package, which is added to the environment files.

* Fixed path to ISIS docs for -WEBHELP

The hardcoded path to ISIS docs was incorrect. The path to docmentation is $ISISROOT/docs - the s was missing in the path spec.