Move showcasegallery content to gallery.md
Move showcasegallery content to gallery.md

uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite