Hide spec from navbar

[stevapple]

[iBug]

We should probably still link to the spec pages somewhere so that they remain accessible.

Linking it from README.md using relative links won't work very well because the spec pages contain mkdocs-material-specific markup that doesn't render on GitHub. The existing link to 101.lug.ustc.edu.cn looks fine to me. (P.S. removing nav items from mkdocs.yml does not stop the markdown files from being rendered.)

[stevapple]

The existing link to 101.lug.ustc.edu.cn looks fine to me. (P.S. removing nav items from mkdocs.yml does not stop the markdown files from being rendered.)

This PR also moves Specs out of docs so it's not rendered.

We should probably still link to the spec pages somewhere so that they remain accessible.

If you'd like to preserve specs pages on the website, maybe we can just remove it from Nav.

[iBug]

I'm inclined to preserve the pages on the website because of the use of theme-specific markup. Also note that some content in the specs relies on such markup (e.g. this). Hiding them from the navbar should suffice.

[iBug]

I wonder if a "Spec landing page" is really necessary. Considering that it's unlikely that we add a 3rd page later on, it may be simpler to just link to both spec pages separately.