Miss `cir::StructType` documentation

[FantasqueX]

There is no cir::StructType in https://llvm.github.io/clangir/Dialect/types.html because it is not generated by ODS.

[smeenai]

We should be able to generate mutable types via ODS now. llvm/llvm-project#117485 (and the follow-up llvm/llvm-project#117930) did so for the LLVM dialect's struct type.