The SHORTCODE.Ordinal method "returns the zero-based ordinal of the shortcode in
relation to its parent." (https://gohugo.io/methods/shortcode/ordinal/)
This is a problem if the same shortcode is used on the different page through
different paths, e.g. once on the page itself and once by a partial (e.g. a
header or footer).
Instead of just using the .Ordinal by itself, use a hash of the parent file
_and_ the ordinal, which gives us a properly unique identifier.