Allow attaching descriptions to OpInterfaces and InterfaceMethods.

This change adds support for documenting interfaces and their methods. A tablegen generator for the interface documentation is also added(gen-op-interface-doc).

Documentation is added to an OpInterface via the `description` field:
def MyOpInterface : OpInterface<"MyOpInterface"> {
  let description = [{
    My interface is very interesting.
  }];
}

Documentation is added to an InterfaceMethod via a new `description` field that comes right before the optional body:

InterfaceMethod<"void", "foo", (ins), [{
  This is the foo method.
}]>,

PiperOrigin-RevId: 270965485