DOC: Update documentation

[fonnesbeck]

Issue with current documentation:

The PyTensor docs have lagged behind development in general, and several developers have expressed interest in updating them. This is a meta-issue for coordinating a collaborative effort for re-writing the documentation.

If you are interested in tackling a particular subset of the docs, please indicate the scope of your intended work in the comments below and link to a PR (even if it is in draft form). Once your work has been merged, please edit the table below to add your updates.

Docs Updated	Name	Date