Allow cross-referencing in nav

[haneul]

Sometimes we want to add a page under different subsites (or main site) in a subsite's nav. While linking with ../ may work, but mkdocs can't properly process as it can't find the page.

It shows error like WARNING - A relative path to 'test/../cross.md' is included in the 'nav' configuration, which is not found in the documentation files

And the navigation title will be None.

This PR fixes it by having normpath for nav link.

[haneul]

okay updated

[agentbellnorm]

Seems like a test needs updated.

[bih]

@haneul Can you go ahead and update the test? I can go ahead and merge this once that's resolved :)