Point project maintainers on how to set up documentation translation

[Данило Шеган]

Some project maintainers want to set up documentation translation using Launchpad. This procedure is relatively similar to setting up a general code project for translation, but involves using tools like gnome-doc-utils (and xml2po) instead of intltool.

Launchpad should promote one official way of doing it (eg. xml2po+gnome-doc-utils), but should not restrict people to it (eg. poxml files should be acceptable as well).

We should have documentation to explain how to set the system up, most importantly for DocBook, and point to it from project translations pages.