Merge: Nitweb clean

Cleans some misc part of Nitweb. Notable fact, replace the hover on cards by a dropdown menu with caret.

Can be seen here: http://nitweb.moz-code.org/doc/core::array

For now, the dropdown menu allows the user to swtich between signature and documentation. More contextual options in the future.

This looks quirk with the stars rating (left untouched in the new dropdown system) so still need the mouse over. Since the next PR rewrite this part, I will let this as it is.
We load the stars only when the dropdown is selected so we gain some perfs on the home page.

This PR is based on #2265

Pull-Request: #2268
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Simon Zeni <simonzeni@gmail.com>