Migrate to the new domain #333

We would like to migrate to a new domain, independent of ExaWorks, to emphasize that PSI/J is not meant to "decay" once ExaWorks (in its current form) ends.

There are some questions as to how that should be done. So this issue is here to track the problems associated with the move and possible solutions.

Like the whole project, this is a community-wide issue, so feel free to assign yourself and/or comment.

One issue is that we have PSI/J the specification and psij-python, the implementation. One idea is to have the PSI/J part live under psij.io/spec [better names welcome] and psij-python under psij.io/python, while having an overview page on psij.io that links to both.