reenable docsite link checking, fix broken links
Created by: sqs
297a6e5841378cd9138ad22878b156d7c86fbdca erroneously told docsite to NOT check for broken links that matched the regexp ^/
, which covers basically all links between documents (because links are checked in the rendered HTML, not Markdown, and the rendered HTML links all prefix the base URL path).
This fixes broken links that crept in and reenables broken link checking for relative links.