Investigate re-enabling `mlir-doc` target

[rolfmorel]

#939 removed the lines of the README regarding being able to build TableGen-derived documentation. The issue is that the mlir-doc target is broken at the moment.

We should investigate the issue (a CMakelists.txt issue?), fix it, and add building this target to the CI. When that's done, we can add back the README lines on generating the documentation.

[adam-smnk]

This probably needs CheckOps.td to be split into a separate dialect TD file. Good opportunity to also do some minor cleanup.

[adam-smnk]

@rengolin Should we add docs to CI? Otherwise, IMO no point in maintaining it at all as we don't really use them.

[rengolin]

The question here is if we want to maintain this at all. If the answer is yes, then we must do it right and add it to CI. Otherwise we just close this issue and don't think about it.