Readme & Web-site update

[mauro3]

Would be great to have the web appearance a bit more polished.

https://ptsolvers.github.io/ is not particularly good

https://ptsolvers.github.io/GPU4GEO is much better. But nice would be to have a somewhat dedicated FastIce page, could be https://ptsolvers.github.io/GPU4GEO/stream/ but with different text.

Cc: @horganhu

[mauro3]

Ok, now is the time. How can I help?