Replace legacy backend docs with clearer guides

[Nicolass67]

Summary

This PR replaces the legacy backend documentation with a new documentation set that is easier to read, more explicit, and better organized.

The previous docs were mainly diagram-based and hard to follow without already knowing the codebase. The new docs explain the backend architecture, request lifecycle, API behavior, conversion flows, prediction flows, and onboarding path in plain language.

What changed

• Replaced the old architecture documentation with a clearer backend architecture guide.
• Added onboarding documentation for developers discovering the project.
• Added API reference documentation for the main backend endpoints.
• Added core flow documentation for conversion, ZIP handling, transformations, and predictions.
• Reworked and regenerated diagrams to make them more readable.

Goal

The goal is to help developers understand the backend faster, debug flows more easily, and make future changes with more confidence.