Add CI job to generate glib frontend HTML-based API documentation.

10 jobs for master in 25 minutes and 2 seconds (queued for 23 seconds)
Status Name Job ID Coverage
  Build
passed build #183754

00:05:05

passed build_android #183759

00:02:54

passed build_clang_libcpp #183755

00:05:53

passed build_clazy #183758

00:06:43

passed build_gcc_49 #183756

00:12:17

passed build_mingw64_fedora #183757

00:06:24

 
  Document
passed cpp_docs #183761

00:05:27

passed glib_docs #183762

00:05:59

passed qt5_docs #183760

00:05:52

 
  Publish
passed trigger_pages #183763

00:00:20