[Utility] Document tx fee logic (valid, invalid, included in block or not)

[jessicadaugherty]

Objective

Document all existing tx business logic in the utility module for guidance on what makes a tx valid, invalid, and how it works with mempool storage and state propagation (including fees).

Origin Document

After refactoring the tx business fee logic in #476 the flow should be documented for reference as additional M3 interfaces are implemented.

Goals

• Provide clear documentation of how the tx business logic works

Deliverables

• Create a read.me to document tx fee logic incuding:
  • Logic for valid txs
  • Logic for invalid txs
  • Logic for fees and pools
  • Logic for storing and validating txs (mempool, in block, etc.)

Non-Goals / Deliverables

• Designing, implementing, or automating tx fee logic tests

General issue checklist

• Update the appropriate CHANGELOG
• Update the README
• If applicable, update the source code tree explanation
• If applicable, add or update a state, sequence or flowchart diagram using mermaid
• Update any relevant global documentation & references
• Document small issues / TODOs along the way

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Creator: @jessicadaugherty