docs: add tensor_ext canonicalization documentation somewhere under the Dialect/TensorExt page

[asraa]

On the website, we can either append it as a section on the single TensorExt page: https://heir.dev/docs/dialects/tensorext/, or we can add a sub-page about Design or more info?

[j2kun]

Canonicalization rules are weird in that, to my knowledge, there's no way to auto-generate docs from the tblgen. So this would have to be a manual process. I'll ask around in the MLIR world to see if there's a better way.

[asraa]

Right! My feeling was putting a manual description in a README.md as a section.

[j2kun]

Posted https://discourse.llvm.org/t/autogenerated-documentation-for-drr-patterns/79426