Add docstrings to all a-frame components and systems

src/components/animation-mixer.js

src/components/audio-feedback.js

src/components/avatar-replay.js

src/components/block-button.js

src/components/bone-mute-state-indicator.js

src/components/bone-visibility.js

src/components/cardboard-controls.js

src/components/character-controller.js

src/components/controls-shape-offset.js

src/components/css-class.js

src/components/cursor-controller.js

src/components/duck.js

src/components/event-repeater.js

src/components/freeze-controller.js

src/components/gltf-bundle.js

src/components/gltf-model-plus.js

src/components/hand-controls2.js

src/components/hand-poses.js

src/components/haptic-feedback.js

src/components/hide-when-quality.js