[Flang][Doc] Add a FIROperations section (#72252)

Adding this section ensures that the documentation generated by
mlir-tblgen for all the FIR operations are at the correct depth.
FIROperations are at depth 3, the new section is at depth 2. This fixes
the "Non-consecutive header level increase; H1 to H3" warning in
FIRLangRef.md.