Add check that relative links are used (no absolute links to handbook.sourcegraph.com)

[felixfbecker]

Using relative links is critical to make sure that the handbook can be browsed on deploy previews in PRs and on old revisions. It can be unintuitive for people not familiar with the concept though, which is why we have this handbook page: https://handbook.sourcegraph.com/editing/linking-within-handbook

Our link checker cannot check for this directly, but we could add a step that simply does a grep for links looking like [...](https://handbook.sourcegraph.com/...) in the content/ directory.