Landing page needs high-level motivation

[cthoyt]

When you got to the SSSOM website (https://mapping-commons.github.io/sssom), it gets right into the technical description. The first page should provide the high-level background and motivation for its existence. This text can be adapted from the README and the manuscript.

[matentzn]

Maybe we should have that page redirect here: https://mapping-commons.github.io/sssom/spec/

And add the "SSSOM elevator pitch" i recorded for someone recently: https://www.youtube.com/watch?feature=shared&v=T-YP-Lv9k2s up top as well as harmonising the high level messaging.