Fix broken links

Running

htmlproofer _public/ --log-level :debug --empty-alt-ignore > report

Then grepping the report for non-200 response codes:

> grep "Received a" report | grep -v " 200 " | sort | uniq

I see this sadly long list of invalid external links from the Graphviz docs:

Received a 0 for http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/lain/lain/docs/Attic/wplug.psql?rev=1.1.1.1&content-type=text/plain  in _public/_pages/Misc/sql2dot/index.html
Received a 0 for http://download.fedora.redhat.com/pub/fedora/linux/extras/  in _public/resources/index.html
Received a 0 for http://infosthetics.com/  in _public/theory/index.html
Received a 0 for http://networkx.lanl.gov/pygraphviz/  in _public/resources/index.html
Received a 0 for http://thyer.name/lambda-animator/  in _public/resources/index.html
Received a 0 for http://www.graphspe.com/#/Home  in _public/resources/index.html
Received a 0 for http://www.portablecomponentsforall.com/edu/graphviz-about-ru/  in _public/index.html
Received a 0 for http://www.vrml.org/  in _public/doc/info/output.html
Received a 0 for http://www.yoix.org/  in _public/resources/index.html
Received a 0 for http://www2.parc.com/istl/projects/uir/projects/ii.html  in _public/theory/index.html
Received a 0 for https://mailman.research.att.com/pipermail/graphviz-devel/2007/000444.html  in _public/doc/winbuild.html
Received a 0 for https://www.antwoorden.org/welkom-bij-graphviz/  in _public/index.html
Received a 0 for https://www.levelsanswers.com/%d0%b4%d0%be%d0%b1%d1%80%d0%b5-%d0%b4%d0%be%d1%88%d0%bb%d0%b8-%d0%b2-%d0%b3%d1%80%d0%b0%d0%b2%d0%b8%d0%b7  in _public/index.html
Received a 0 for https://www.losungenapp.com/willkommen-in-graphviz/  in _public/index.html
Received a 0 for https://www.solutionjeux.info/bienvenue-chez-graphviz/  in _public/index.html
Received a 301 for http://www.math.yorku.ca/SCS/StatResource.html  in _public/theory/index.html
Received a 302 for http://pdx.freedesktop.org/~fontconfig/fontconfig-user.html  in _public/doc/info/attrs.html
Received a 400 for http://www2.research.att.com/~volinsky/cgi-bin/prox/prox.pl  in _public/resources/index.html
Received a 400 for http://www2.research.att.com/~yifanhu/GALLERY/GRAPHS/index.html  in _public/_pages/Gallery/undirected/root.html
Received a 400 for http://www2.research.att.com/~yifanhu/MAPS/index.html  in _public/_pages/Gallery/undirected/gd_1994_2007.html
Received a 400 for http://www2.research.att.com/~yifanhu/contact.html  in _public/_pages/Gallery/gradient/gradient.html
Received a 403 for http://drupal.org/project/graphapi/  in _public/resources/index.html
Received a 403 for http://www.stack.nl/%7Edimitri/doxygen/index.html  in _public/about/index.html
Received a 404 for http://cs.brown.edu/~rt/gdhandbook/  in _public/theory/index.html
Received a 404 for http://lain.sourceforge.net  in _public/_pages/Misc/sql2dot/index.html
Received a 404 for http://lnkd.in/4-p_zi  in _public/resources/index.html
Received a 404 for http://ociotec.com/index.php/2008/02/25/eht-graphviz-plugin-para-wordpress/  in _public/resources/index.html
Received a 404 for http://projects.gnome.org/gtkglext/  in _public/_pages/Download/Download_source.html
Received a 404 for http://quickgraph.tigris.org/source/browse/quickgraph/trunk/src/  in _public/resources/index.html
Received a 404 for http://techion.com.au/blog/2014/8/17/creating-os-x-package-files-pkg-in-terminal  in _public/download/index.html
Received a 404 for http://www.fontconfig.org/wiki/  in _public/_pages/Download/Download_source.html
Received a 404 for http://www.graphviz.org/Download_macos.php  in _public/doc/build.html
Received a 404 for http://www.graphviz.org/Download_source.php  in _public/doc/winbuild.html
Received a 404 for http://www.graphviz.org/Download_windows.php  in _public/doc/winbuild.html
Received a 404 for http://www.graphviz.org/Misc/third-party.zip  in _public/doc/winbuild.html
Received a 404 for http://www.gzip.org/zlib/  in _public/doc/winbuild.html
Received a 404 for http://www.hotscripts.com/listing/php-traverser/  in _public/resources/index.html
Received a 404 for http://www.mathematica-journal.com/issue/v10i1/graph_draw.html  in _public/theory/index.html
Received a 404 for http://www.merl.com/papers/TR2001-49/  in _public/theory/index.html
Received a 404 for http://www.worldcat.org/xissn/titlehistory  in _public/resources/index.html
Received a 404 for https://marketplace.visualstudio.com/items?itemName=joaompinto.vscode-graphviz  in _public/resources/index.html
Received a 500 for http://code.google.com/p/graphviznet/  in _public/resources/index.html
Received a 500 for http://code.google.com/p/nodeviz/  in _public/resources/index.html
Received a 500 for https://developer.apple.com/library/mac/documentation/graphicsimaging/reference/CGImage/Reference/reference.html  in _public/doc/info/output.html

Possibly not all of these are legit (particularly some of the 0 status codes could be network problems), but many will be.