Add Gravity case

[marcoow]

This adds Gravity as a case study.

TODO

• add og:image
• add better background image for the header
• add some kind of screenshot of the landing page (see TODO)
• add video of Gravity (see TODO)
• add CTA for Gravity (see Gravity announcement #2643)

[oscard0m]

can we position the video nicely? It looks a bit randomly positioned like this:

It's much nicer on the blog – guess we can just reproduce that here.

applied missing class to have the same look and feel