Add tests for menu links

On Nanoc, the global navigation is generated by Nanoc and output as HTML via the Nanoc templating system. This means the menu links are testable by any tool that can check regular HTML, like Lychee.

On this project, we've done the global navigation in Vue. This has a few benefits (better performance, significantly smaller build size, easier templating), but this does mean the rendered menu is not part of the HTML files outside of the browser context. How can we test that these URLs resolve?

All tests required here:

1. Something new to validate links in the global nav
2. pages_not_in_nav.js -- !155 (merged)
3. YAML schema validation for navigation.yaml -- !155 (merged)
4. check-navigation.sh -- !155 (merged)