help: move medialibrary.page to index#gettingstarted
help: move welcomedialog.page to index#intro
help: move mainwindow.page to index#intro and delete interface.page
help: overall improvements in effects.page
help: overall improvements in keyframecurves.page
help: overall improvements in trimming.page
help: rename effectsanimation.page to usingeffects.page
help: move Configuring effects section from effects.page to usingeffects.page
help: add Effect Library section to usingeffects.page
help: add Animating effects with keyframes section to usingeffects.page
help: update meta info in usingeffects.page
help: create transitions.page
help: copy section from crossfading.page to transitions.page and update it
help: copy section from fadeinfadeout.page to transitions.page and update it
help: add Other types of transitions section to transitions.page
help: remove crossfading.page, fadeinfadeout.page and othertransitions.page
help: reword and update codecscontainers.page
help: reword and update gstreamer.page
help: delete the Marker bar section in mainwindow.page
help: merge Special Effects and Transitions sections in index.page