Doc: footer help message for translated manuals

[Federico Bruni]

Considering the frequent reports sent in this list about errors in the 
german documentation and then forwarded to the translation list, what 
about adding a link to the translation list in the doc footer?

Currently the footer is:
We welcome your aid; please help us by reporting errors to our bug list.

It can be changed to:
We welcome your aid; please report errors in the english documentation 
to our bug list and errors in any other language to our translation 
list.

The footer is defined in python/auxiliar/postprocess_html.py