Fixes some issues in the documentation related to external hyperlinks

* Fixes bugs in the definition of `\fnurl`:
  * URLs containing special characters (e.g. `#`) were not correctly protected what led to compiling errors.
  * The argument of `\fnurl`, e.g. `\fnurl{\biblatexctan doc/examples}`, got not expanded for the label of the link.
* Fixes double slash error in link to Biblatex examples folder on CTAN.