Beautify "Overview" page

Currently, the Overview page is nice but may be too technical for someone wanting to grasp the main features of Orekit at a glance.

It should be improved to make it more appealing to a non-Flight Dynamics expert.

The "Features" part, for example, is very technical and is more or less a copy-paste of the same page in the technical site.

A (non-contractual 😉) example we should try to follow could be the Gitlab product page.