docs: add assumptions made by off-chain services

[varunsrin]

Motivation

Add docs clarifying the assumptions that off-chain services make on the contracts

Change Summary

See above

Merge Checklist

• The PR title adheres to the conventional commits standard
• The PR has been tagged with change type label(s) (i.e. documentation, feature, bugfix, or chore)
• The PR's changes adhere to all the requirements in the contribution guidelines
• This PR does not require changes to the Farcaster protocol

---

PR-Codex overview

Detailed summary

• Added documentation for the L2 contracts: Id Registry, Storage Registry, Key Registry, and Bundler.
• Added documentation for the L1 contract: Fname Resolver.
• Added documentation for the off-chain systems: Fname Registry and Hubs.

✨ Ask PR-Codex anything about this PR by commenting with /codex {your question}

[github-actions]

Coverage after merging varunsrin/document-offchain-assumptions into main will be

98.70%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src
Bundler.sol	100%	100%	100%	100%
FnameResolver.sol	100%	100%	100%	100%
IdRegistry.sol	100%	100%	100%	100%
KeyRegistry.sol	100%	100%	100%	100%
StorageRegistry.sol	100%	100%	100%	100%
src/lib
Signatures.sol	100%	100%	100%	100%
TransferHelper.sol	0%	0%	0%	0%	15, 15, 15, 25–26, 26, 26
TrustedCaller.sol	100%	100%	100%	100%