Add opengraph metadata for decomp.me homepage

[mkst]

Currently links to decomp.me itself are pretty boring, e.g.

Describe the solution you'd like Off the back of the improved opengraph image for individual scratches, we should take the opportunity to provide a good embed for anyone creating a link to decomp.me itself.

decomp.me A collaborative reverse-engineering platform for working on decompilation projects with others to learn about how your favorite games work. 123,456 unique scratches created to date.

We could use opengraph-image here too if we came up with a design :)