Move https://turing.mybinder.org/ to alumni

[rgaiacs]

Bug description

https://mybinder.readthedocs.io/en/latest/about/federation.html lists https://turing.mybinder.org/

but https://turing.mybinder.org/ returns

Could not resolve host: turing.mybinder.org

because Turing decide to retire their servers as reported in jupyterhub/mybinder.org-deploy#2396.

Expected behaviour

Links in https://mybinder.readthedocs.io/en/latest/about/federation.html are valid.

Actual behaviour

https://turing.mybinder.org/ is invalid.

How to reproduce

1. Visit https://turing.mybinder.org/

Suggestion

Should we create a alumni section?

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