Mention XTensorVariable methods in dims module

[ricardoV94]

Description

The math page is just a reference point to where on the pytensor docs the related functionality is documented. Many operations though are available as methods of XTensorVariables, like .isel and .dot, and we should mention and link:

https://www.pymc.io/projects/docs/en/stable/api/dims/math.html
https://pytensor.readthedocs.io/en/latest/library/xtensor/type.html#pytensor.xtensor.type.XTensorVariable

[ricardoV94]

Also title is too verbose, le't just go with `Mathematical operations'