[mlir] fix IRDL dialect docs (#92349)

Currently the irdl dialect page has no content beyond the header.

By referring to the Ops.td in the CMake config, it pulls in all the
types, attributes, etc., so that the doc generation can include them all
in the page.

Rendered locally to confirm it fixes the issue


![image](https://github.com/llvm/llvm-project/assets/2467754/8758f324-6bc3-4f0e-8fa9-8962cdb0177f

)

Co-authored-by: Jeremy Kun <j2kun@users.noreply.github.com>