DOC: Remove & replace links to the (nonexistent) GitHub Wiki

[michaelosthege]

There are a few links to GitHub Wiki pages which were not migrated by forking.

They can be found by search: https://github.com/pymc-devs/pytensor/search?q=wiki

Tasks

• Check the corresponding wiki pages in the Aesara project
• Remove links or migrate content to pages in the PyTensor documentation, or link to equivalent PyMC documentation

[ricardoV94]

There is also some messages that refer to Theano mailing list