add guide.sgml and offline.sgml to the "translatable with po4a"-group

add guide.sgml and offline.sgml to the "translatable with po4a"-group

Fix also the buildsystem for building the coresponding docs from these
two files if they are exist for this language so we don't need to change
the makefiles for ja and fr - and can also use the same simple makefiles
for it and pl which ship these files as not-yet-with-po4a-translated
(and currently no man page translation.)