subdomain for snapatac2

[grst]

frameworks typically have their own subdomain, e.g. spatialdata.scverse.org, scirpy.scverse.org

SnapATAC2 is currently https://scverse.org/SnapATAC2/

[grst]

While we are at it, we could also setup subdomains for packages that currently exclusively rely on readthedocs, e.g.

https://decoupler-py.readthedocs.io/en/latest/ -> decoupler.scverse.org