Tidy up docs home page

[drbenvincent]

There was a recent refresh of the pymc-marketing docs homepage, and it's looking quite nice.

Let's make the following changes:

• Add download stats badges (@juanitorduz might be interested in doing this?)
• Change the Features section
  • Remove all the images
  • Add a table to outline all the experiment types. Perhaps having ticks/crosses for Bayes and OLS versions.