Fix dead links on migrated website (Error 404 etc)

We know about a few problem links e.g. #8 #13 #17, but should check this systematically. There are also some external links which no longer work.

Ideally we'd have a TravisCI configuration to run Jekyll and then spider the `_site/` folder to spot any link breakage.

Start with the known problem links in issues #8, #13, and #17, and review how Jekyll generates the `_site/` folder. Determine how a TravisCI configuration could build the site and spider both internal and external links. Done means the existing broken links are addressed and automated checking can detect future link failures.

Written by the indexing model from the issue text.