Add a notice that the docs on readthedocs.io are outdated

[yut23]

The old docs at https://pyro2.readthedocs.io/en/latest/ should have a pointer (or redirect, if possible) to https://python-hydro.github.io/pyro2/

[zingale]

unfortunately, I don't have access to the readthedocs account for pyro -- someone else created it.

[zingale]

@IanHawke do you have access to the RTD page? Is it possible to disable the pyro docs? We've switch back to a github action since we have more control over the build.