Add a section on tx mempool behavior in Consensus Subprotocols page

[dariolina]

• Conditions on which txs are accepted to mempool
• Conditions on which txs are not accepted
• Conditions on which txs are first accepted and later dropped (i.e. fees)
• Size limitations
• Behavior when mempool is full

Most of mempool default behaviors are inherited from Substrate, but we want fraud proofs to have separate handling (see autonomys/subspace#2570 and autonomys/subspace#2567)