M3(P2O7)22‐‐Type Open Frameworks Featuring [M2O8] and [M3O12] Multinuclear Transition‐Metal Oxide Units. Serendipitous Synthesis of Six Polymorphic Salt‐Inclusion Magnetic Solids: Na2M3 (P2O7)2·ACl (M: Mn, Fe; A: Rb, Cs) and K2M3 (P2O7)2·CsCl (M: Fe, Mn).