Add automated check for broken links across about.sourcegraph.com

[emidoots]

I spotted a broken link to docs and fixed it in https://github.com/sourcegraph/about/pull/482 -- but there is a bigger issue here: we don't have anything in place to catch broken links or links that lead to a 404 on our about.sourcegraph.com site. More of these could be lurking and we would not know.

[sqs]

I think we should just use an off-the-shelf link checker for that so that it also includes links in the .tsx files in the about website, not just the handbook. (I.e., not build this into docsite in case anyone was thinking that.) Not sure of priority for this. https://sourcegraph.slack.com/archives/CQ44Y7F4G/p1602609602046800?thread_ts=1602609128.044600&cid=CQ44Y7F4G

[aileenrose]

I will look into this. Thanks.