Answer Blockchain questions

[Prebenha]

Write a answers to the questions for the subject Blockchain.

The answers should be highly relevant to the DAT159 course.

The file should be a markdown file and be named index.md located in:

pages/expr/questions/blockchain/index.md

[181192]

Great work @181394!
A couple of questions left, if someone else wants to keep going:

What is known as the biggest risk of distributed consensus?
What are inputs and outputs?
What are transactions, signing, validation?
What is private key, public key, address and signature?
What is hashing used for?
What is the UTXO-set?
What is mining, nonce and "target"?
What is the difference between PoW and PoS?
What is the consequence if a block on the chain is altered after it's been added?