Documentation Link Checker

[andymandias]

To help catch link errors in the documentation this adds mdbook-linkcheck to mdbook output. Running it locally caught a few errors (including recently found broken link fixed by #780), which are fixed in this PR as well.

The changes to GH workflows are due to this note:

When multiple [output] items are specified, mdbook tries to ensure that each [output] gets its own sub-directory within the build-dir (book/ by default).
That means if you go from only having the HTML renderer enabled to enabling both HTML and the linkchecker, your HTML will be placed in book/html/ instead of just book/ like before.

But, I'm not sure how to test the change.