Using Olivier's idea to check the meta tag, we can distinguish rustdoc
from "the rest". The html is included in mdbook by directly by
symlinking head.hbs.
The preview is now in sixtyfps-docs-preview.html and the highlighting in
sixtyfps-docs-highlight.html. This faciliates reusing the preview from
C++ while using a different HTML file later for syntax highlighting.
Relates to #282
Needed for #281