Add internal link checking to our Jekyll sites

[PeterJCLaw]

It's currently far too easy to typo a link (or remove a page) and end up with broken links in sites. While these can be checked manually, it should also be possible to automate this. We'd likely want to ignore links to external pages, but that should be easy enough.

Sites:

• website (Use html-proofer to validate links (etc.) website#320)
• docs (Add html-proofer as a validation mechanism docs#215)
• style (Add html-proofer for link validation style#24)
• competition-website (Add html-proofer for link validation competition-website#25)

[Tyler-Ward]

see also srobo/docs#213

[PeterJCLaw]

I had a quick play with some tools:

• https://pypi.org/project/link-checker/ seems promising but is Python and Jekyll is Ruby, so it would be nice not to need another language runtime. This might be a good fit for the runbook though.
• https://github.com/gjtorikian/html-proofer seems promising, but does way more than just link checking. It seems pretty configurable, however does have some issues that would create problems for us (notably Add support for site.baseurl, workaround gjtorikian/html-proofer#618 and mailto without email shouldn't fail gjtorikian/html-proofer#552)

[PeterJCLaw]

This is now done (for Jekyll sites).
There's obviously plenty this doesn't cover -- notably cross-site links or non-Jekyll things (like the runbook), but we can pick those up separately.