Adding linkcheck (and check for other build problems)

[bsipocz]

This will close #156

I expect seeing broken links here which should be fixed.

[rossbar]

Great - thanks @bsipocz ! This is a definite improvement

For whatever reason the CI run borked the first time through - when I re-ran it it failed as expected (including the errors in the log). I took the liberty of pushing up fixes for those errors. Most of them were either removing or replacing dead links (see individual commits for comments).

The last one is the latex rendering error which has been present for a while. I finally went about searching how to fix it :) - the build errors are gone and the rendering now seems to be fixed (scroll down a bit from that anchor to see the rendered TeX - compare with what's currently deployed).

Circleci in particular seems a bit flaky (there have been a couple timeouts related to this PR) - but there's no evidence it's related to the --strict. My vote would be to put this in and simply keep an eye on circleci to see if we continue getting extraneous timeouts.

Thanks again @bsipocz ! I approve this but will let someone else merge so folks have a chance to look the changes I pushed up too!