Upgrade jQuery to 3.x

[Andžs]

docDock currently use jQuery 2.x, which is deprecated. To upgrade 3.x, one should:

1. Update all links to jQuery on CDN
2. Check for errors in the browser console
3. Check for errors with main functions/features from custom docDock functions
4. Check for errors with docDock "plugins" like reveal.js etc.

PR must be against DEV branch so results could be verified also by others.

Optional webcomic on this topic.

Image from GARABATOKID