Add non-github repo for graph-tool

[jbednar]

Attempt to add graph-tool, even though it has no Github repo (see #59). There will probably be some broken image links (e.g. no github stars), but maybe it will otherwise work? We'll see what happens at https://pyviz-dev.github.io/website/tools.html

[jbednar]

Looks like it works about as well as can be expected:

I.e., links that point to github don't work, but the rest are fine. I'll go ahead and merge this as better than nothing, as it does at least allow listing graph-tool in the overall page format. I'll leave #59 open and add comments there for anyone who wants to improve what happens for this case.