docs: add calculations logic doc

[muchai254]

This PR adds documentation that explains How frontend nodes and backend calculation logic fit together. The following topics are covered:

1. What Is a Calculation Node?
2. The Lifecycle of a Calculation
3. How the Backend Resolves Inputs
4. A POST /bulk_calculate Request and Response Example
5. A Concrete Example: P2PKH Address Derivation
6. Nodes With Multiple Output Handles
7. Where Things are in the Codebase
8. Common Failure Cases

[rawBit-io]

Thanks for the work on this. I’m not merging it in this form: it’s too long for our docs and has some accuracy issues. Please replace it with a much shorter, high-level contributor note only.