Broken links checking

[begleynk]

We should add a feature where we verify that internal links within a project are not broken.

Current plan would be that when running doctave build, we'd show any errors and fail the build if there are broken links, with a flag that allows for ignoring the error status.

I'm going to narrow this down to internal links only for now, as going over the network to check external links has lots of additional complexity (servers can go down, some internal systems may not be accessible from CI, etc).