Document draft Token Bridge contract

[critesjosh]

Document the token bridge contract.

Here is the tutorial.

[critesjosh]

I believe @rahul-kothari mentioned this in another issue, but I can't find it right now.

[rahul-kothari]

#61 is more on creating the whole e2e setup with hardhat, solidity contracts, aztec.js and aztec-nr. My work on #58 was just the aztec-nr explanation.