Add section in consensus protocol for block finality calculation

[lparisc]

The actual time required for a transaction within a block to become final in the blockchain is a recurring theme. Currently, this calculation is missing from the documentation.

Perhaps a good place to explain this is in the Consensus Protocol within the 5.4. Block Finality section, here: https://docs.eosnetwork.com/docs/latest/protocol-guides/consensus-protocol#54-block-finality, right after all production-related concepts have been explained in 4.2. Producers schedule and 5. Block Lifecycle.

AC:

• Add new paragraph under 5.4. Block Finality to go over the actual block finality time calculation.

[lparisc]

Currently, each block becomes final in 180 seconds:

• 2 rounds x (15 producers x 12 blocks/producer) per round x 0.5 secs/block = 180 secs
• The 15 producers is the supermajority (2/3+1) of the 21 producers

source: https://eosio.stackexchange.com/questions/445/how-long-on-average-before-a-transaction-included-in-a-block-is-final-and-irreve