Deprecate/refactor content in docs/specification (#1934)

* docs: deprecate specification dir, closes #1814

* update genesis

* old spec dir, deprecation complete

* rm a file

docs/tendermint-core/validators.md

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+# Validators
+
+Validators are responsible for committing new blocks in the blockchain.
+These validators participate in the consensus protocol by broadcasting
+*votes* which contain cryptographic signatures signed by each
+validator's private key.
+
+Some Proof-of-Stake consensus algorithms aim to create a "completely"
+decentralized system where all stakeholders (even those who are not
+always available online) participate in the committing of blocks.
+Tendermint has a different approach to block creation. Validators are
+expected to be online, and the set of validators is permissioned/curated
+by some external process. Proof-of-stake is not required, but can be
+implemented on top of Tendermint consensus. That is, validators may be
+required to post collateral on-chain, off-chain, or may not be required
+to post any collateral at all.
+
+Validators have a cryptographic key-pair and an associated amount of
+"voting power". Voting power need not be the same.
+
+## Becoming a Validator
+
+There are two ways to become validator.
+
+1.  They can be pre-established in the [genesis state](../../tendermint-core/using-tendermint.md#genesis)
+2.  The ABCI app responds to the EndBlock message with changes to the
+    existing validator set.
+
+## Committing a Block
+
+*+2/3 is short for "more than 2/3"*
+
+A block is committed when +2/3 of the validator set sign [precommit
+votes](../spec/blockchain/blockchain.md#vote) for that block at the same `round`.
+The +2/3 set of precommit votes is called a
+[*commit*](../spec/blockchain/blockchain.md#commit). While any +2/3 set of
+precommits for the same block at the same height&round can serve as
+validation, the canonical commit is included in the next block (see
+[LastCommit](../spec/blockchain/blockchain.md#last-commit)).