Update page variable list for custom taxonomies

I love the new support for custom taxonomies! I got a little thrown off when updating my site, though, as the docs didn't specify how the page would expose the taxonomies to the template. Wasn't too hard to figure it out, but I figured I'd save the next person who tries it the effort :)

Section assets

This reverts commit c7156a84f0.

Fix path handling on windows

Otherwise Path won't know what to do.

Delay turning paths into string. Instead, use methods on Path/PathBuf
to manipulate paths. This way the Rust Path implementation gets to deal with
all werid cross-platform issues.

This should fix #359

Next version

Remove `order` and add `heavier`/`later`

Replace `populate_next_and_previous_page` with `populate_siblings`