Document part of consensus rules from Zcash spec: 7.6 Block Header Encoding and Consensus

[mpguerra]

For each consensus rule:

• check that the rule is implemented in Zebra
• make sure the latest version of the rule is quoted in the docs for the code that implements it
• make sure the code implements the latest version of the rule

Consensus rule

https://zips.z.cash/protocol/protocol.pdf#blockheader

Implementation

Implemented in:

• Tracking: block header validation #477
  • Verify that the Equihash Solution is valid for the Block Header #591 (PR: Add check for header solution and test case #607)

[oxarbitrage]

Yes i divided the ticket. This one is now for "everything else" and #3297 is for the root stuff. I changed the screenshot of the spec in this ticket to reflect this.