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