Attempt at supporting superscipt and subscript

[romen]

XSLT was removing superscript/subscript, this commit is a first shot at trying to let them pass through the transformation to HTML.

https://docutils.sourceforge.io/docs/ref/rst/roles.html#superscript

I am not too familiar with XSLT, so this is just a hackish attempt, maybe there are more proper ways to solve this.

I welcome any feedback about it!