Generate email for docbook authors

Preferred form for authors in kde docbooks is <author> &author.name;
author.name.mail; </author>, where the entifies are defined in
kdelibs/kdoctools/customization/entities/contributor.entities.
But using this form does not generate the email in the html output without this patch
it is missing apparently since several years (around 2004)

REVIEW:101921

(cherry picked from commit 9c48f7234c42c69909a4a12f013c72ffde77b1c9)