[mlir][docgen] Add ops source link (#73657)

This patch suggests to change two things. Firstly, it adds a source link
above the generated operations docs (above the `emitOpDoc` calls). This
link will point directly to the source TableGen file for the group of
operations. For example, for the current
[`amdgpu`](https://mlir.llvm.org/docs/Dialects/AMDGPU/) page, the link
will add a source link below the "Operation definition" heading pointing
to
[`mlir/include/mlir/Dialect/AMDGPU/IR/AMDGPU.td`](https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/Dialect/AMDGPU/IR/AMDGPU.td).
The link is wrapped in a "op-definitions-source-link" class which could
allow for custom styling, but it also looks reasonable without custom
styling I think:

![afbeelding](https://github.com/llvm/llvm-project/assets/20724914/7c0e59b9-b14b-4f5d-a671-c87e857a7b03)

Secondly, this patch simplifies the header names such as "Operation
definition" and "Attribute definition" to "Operations" and "Attributes"
respec...