all: builders and TryBots should check documentation for broken links

We occasionally end up with broken or outdated links in documentation (see #46545, https://github.com/golang/go/issues/37042#issuecomment-582448476, #27860, #21951, #19244).

Testing for broken links would not be entirely trivial, but nonetheless pretty straightforward to implement: we have an HTML parser in golang.org/x/net/html, and it is easy enough to issue a HEAD request for link targets to see if they resolve. (For links with anchors, we would probably want to also check the Content-Type header and then parse the actual linked HTML to ensure that the anchor exists.)

CC @golang/osp-team