Documentation updates

Adds a lot of new information to the docs that will help developers better utilize the entire Chart.js library

List of implemented changes:
 - Reverse option for legends (resolves #3102)
 - Information about chart resizing (resolves #3023)
 - Mixed chart types (resolves #2825) (resolves #2431)
 - Chart library comparison table (resolves #2605)
 - getDetasetMeta function information (resolves #2521)
 - Popular Extensions (resolves #2365)