Reorganize resources page and make it easier to navigate

[deepthi]

We have a bunch of videos and presentations on this page https://vitess.io/docs/learning-resources/ It's not very easy to navigate. It will be nice to lay the content out in some fashion that doesn't need you to scroll forever. Also, we used to have a right-nav to jump to each section, and that seems to have somehow got lost when we merged #1793