Mention xtensorvariable methods in dims module

[MedAmineGBerry]

Description

This PR enhances the dims math documentation by adding information about the method-based API available on XTensorVariable instances. Previously, the documentation only mentioned where PyTensor operations were wrapped, but didn't inform users that many operations are also available as convenient methods directly on variables.

The addition includes:

• A new section explaining the method-based API on XTensorVariable
• Direct links to key methods like .isel() and .dot() as specifically requested
• Additional commonly used methods grouped by category (aggregation, shape operations)
• A practical example showing how to use these methods with named dimensions

Related Issue

• [x] Closes #7863

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Added necessary documentation (docstrings and/or example notebooks)

Type of change

• [x] Documentation

Notes

Documentation was tested locally. It did generate some folders that I had to delete for the pre-commit to pass. Maybe we should add it to contribution to documentation.