new:dev: many updates to the course

new:dev: remove more errors

new:dev: avoid refs to be included twice so the duplicated label warn…

…ing disappears

fix:usr: github reference card url has changed, so we updated it

new:usr: add videos to the github lesson

chg: dev: Update changelog

new:usr: added a 2 part video about the Web page structure

new:usr: added the first three videos to introduce the class content

new:dev: add contributors

chg: dev: Update changelog