gnu: doc: Allow documentation to be translated.

* po/doc/contributing.pot: New file.
* po/doc/guix.pot: New file.
* po/doc/local.mk: New file.
* Makefile.am: Include it. Add gettext command. Add silent rules for po4a.
* configure.ac: Look for po4a-translate and po4a-updatepo.
* doc/local.mk: Add rules to generate translated texi files.
(TRANSLATED_INFO): New variable.
(BUILT_SOURCES, EXTRA_DIST, MAINTAINERCLEANFILES): Add it.
* .gitignore: Add generated files.