Reorg Consensus (#5780)

* moved page to architecture

* moved questions to faq

* fix broken link

* reorg content

* rearranged text

* added BEEFY key

* final edits

- grammar
- minor additions

* Update docs/learn/learn-consensus.md

* Update docs/learn/learn-consensus.md

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Co-authored-by: Radha <[email protected]>

docs/general/faq.md

@@ -45,6 +45,37 @@ DOT.
 For more information on the Polkadot roadmap please visit the
 [official Polkadot website](https://polkadot.network/technology/#roadmap).
 
+## Consensus
+
+### Why do we need Consensus?
+
+Consensus is a method for coming to agreement over a shared state. In order for the state of the
+blockchain to continue to build and move forward, all nodes in the network must agree and come to
+consensus. It is the way that the nodes in a decentralized network are able to stay synced with each
+other. Without consensus for the decentralized network of nodes in a blockchain, there is no way to
+ensure that the state one node believes is true will be shared by the other nodes. Consensus aims to
+provide the _objective_ view of the state amid participants who each have their own _subjective_
+views of the network. It is the process by which these nodes communicate and come to agreement, and
+are able to build new blocks.
+
+### What are PoW and PoS?
+
+Proof of Work (PoW) and Proof of Stake (PoS) have been inaccurately used as short hand to refer to
+consensus mechanisms of blockchains, but that does not capture the full picture. PoW is the method
+for agreeing on a block author and part of the fuller
+[Nakamoto consensus](../learn/learn-consensus.md#nakamoto-consensus) that also encompasses a chain
+selection algorithm (longest chain rule in Bitcoin). Similarly, PoS is a set of rules for selecting
+the validator set and does not specify a chain selection rule or how a chain might reach finality.
+PoS algorithms have traditionally been paired with an algorithm for coming to Byzantine agreement
+between nodes. For example, [Tendermint](../learn/learn-comparisons-cosmos.md) is a practical
+Byzantine fault tolerant algorithm that uses PoS as its validator set selection method.
+
+### Why not Proof of Work?
+
+Although simple and effective in coming to a decentralized consensus on the next block producer,
+proof of work with Nakamoto consensus consumes an incredible amount of energy, has no economic or
+provable finality, and has no effective strategy in resisting cartels.
+
 ## Validators
 
 ### How do I apply to be a validator?