[mkdocs] docs: add consensus and harvesting concepts

[zero4862]

Adapts Consensus and Harvesting concept to NEM technology.

Differences with Symbol:

Consensus

• Finalization: Not supported on NEM, there's a rewrite limit instead.
• Voting nodes: Not applicable on NEM.
• Receipts: Not supported on NEM.

Harvesting

• Hit calculation: NEM uses a deterministic hash of (parent generation hash, harvester public key). Symbol uses a VRF over the parent generation hash and the harvester's VRF key.
• Harvesting modes: Two on NEM (local, delegated). Three on Symbol (local, remote, delegated).
• Delegation primitive: One ImportanceTransferTransaction on NEM. Symbol uses AccountKeyLinkTransactionV1 plus PersistentDelegationRequest.
• Minimum balance: 10,000 XEM vested on NEM. 10,000 XYM on Symbol (no vesting concept).
• PoI inputs: Vested balance plus a PageRank-like outlink score on NEM. Stake, transaction, and node scores on Symbol.
• Recalculation cadence: Every 359 blocks on NEM. Every 720 blocks on Symbol.
• Rewards composition: Transaction fees only on NEM (no inflation) + Super nodes program. Fees plus inflation on Symbol.
• Rewards routing: 100% to the harvester (or lessor) on NEM. Symbol splits 5% to a network sink, 25% to a node beneficiary, rest to the harvester.

[zero4862]

In NEM, the block is prepared after verifying that the hit < target.

In Symbol, we say the block is prepared before checking hit < target. We should review the order in Symbol in case it is reversed.

[segfaultxavi]

In Symbol the block must be prepared before the hit is calculated, because the block's hash is used to compute the hit, no?

[zero4862]

Added this sentence vs Symbol docs:

* "The block signature comes from the claimed harvester."

And removed "The target and hit values were correctly calculated" since is it covered already by "The hit is indeed lower than the target."

[segfaultxavi]

It's not only about hit being lower than target, it's also about the hit being correctly calculated from the block's hash, the VRF, etc. So I think that sentence is OK in Symbol.
I'll add the point about the signature to the Symbol docs 👍

[segfaultxavi]

I've added a couple minor changes, and it's good to go!

[segfaultxavi]

I like to use the ./ path even for sibling pages, in case some day we enable the plugin that automatically locates pages when they have no path.

[segfaultxavi]

In Symbol the block must be prepared before the hit is calculated, because the block's hash is used to compute the hit, no?

[segfaultxavi]

It's not only about hit being lower than target, it's also about the hit being correctly calculated from the block's hash, the VRF, etc. So I think that sentence is OK in Symbol.
I'll add the point about the signature to the Symbol docs 👍