Combine champlain and champlain-gtk documentations into one

It makes no sense to keep them separate (especially when there's almost
nothing in champlain-gtk). Building documentation requires gtk enabled now.
3 jobs for master in 5 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #210510
build-champlain

00:02:55

 
  Deploy
passed #210511
pages

00:02:32

passed pages:deploy

00:00:01