Fix documentation generation using meson

Meson build removed "lib" prefix from the docs directory name. Because
of this, also files like -sections.txt have to drop the "lib" prefix,
otherwise they are ignored when generating documentation.

Also use xml mode for generating docs and rename sgml file to xml.
4 jobs for master in 11 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #209815
build-champlain

00:02:54

passed #209816
build-champlain-gtk

00:02:31

 
  Deploy
passed #209817
pages

00:08:08

passed pages:deploy

00:00:00