Fix the ability to generate call graph images with Webgrind and graphviz.

Webgrind's default config sets the path for graphviz to /usr/local/bin/dot, and during provisioning, apt-get installs graphviz to /usr/bin/dot. So the proposal here is to simply symlink /usr/local/bin/dot to /usr/bin/dot -- without which you can't currently generate call graphs in Webgrind.
Update docs around Graphviz symlink
Merge branch 'pr/463'

Fixes #463