Broken links should fail the build

[hojberg]

When building the website, if any of the links to other docs (not links to definitions like terms or types) results in a hash or is in other ways malformed, the build should fail, such that we are notified about broken links before they go live.

Note that we might currently be relying on hashes as a way to link to share, meaning, when we see a hash in a link we assume its a link to share. We'd need to differentiate navigational doc links and definition links in some way to catch hashes as errors.

cc @rlmark