Problem: Travis doesn't run linkcheck on documentation

[sallain]

Please describe the problem you'd like to be solved.
Sphinx has a nice little feature called linkcheck that will test out all the external links in the documentation and spit out a report of all the errors. It's helpful, but requires users to remember to run it manually.

Describe the solution you'd like to see implemented.
Include linkcheck as part of the standard Travis CI build for documentation.

Linkcheck is pretty heavy-handed so the fail conditions would need to be considered; we use some dummy links that get caught, for example. As per @ross-spencer's comment in #630, it's possible to create exclusion regex patterns to ignore certain links: http://www.sphinx-doc.org/en/1.2/config.html#confval-linkcheck_ignore.

See also this PR for more examples where valid links get caught by linkcheck: artefactual/archivematica-docs#275

Describe alternatives you've considered.

Additional context
Originally filed by @ross-spencer as https://github.com/artefactual/archivematica-docs/issues/169

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[eviau-artefactual]

I believe that linkcheck is not available as a pre-commit... but I tried adding it to the tests here: artefactual/archivematica-docs#433

[eviau-artefactual]

The linkcheck was implemented in artefactual/archivematica-docs#439

It should work starting for versions 1.15 and up.

Thank you for filing this issue ✨