Add README.md for chainlearn-docs package

[DeFiVC]

What

The chainlearn-docs directory lacks a README.md file explaining its structure and how to use the documentation.

Why

Contributors need to understand:

• How documentation is organized
• How to add new docs
• How ADRs (Architecture Decision Records) work
• How to contribute diagrams

Scope

Create a README.md that covers:

• Directory structure overview
• How to add new documentation
• ADR template and process
• Diagram tooling (Excalidraw)

Acceptance Criteria

• README.md exists in chainlearn-docs/
• Directory structure is explained
• ADR process is documented
• Contribution guidelines are included
• Links to existing docs are provided

Technical Context

• Directory structure:
  • decisions/ - ADRs
  • guides/ - Developer guides
  • contracts/ - Smart contract docs
  • architecture/ - System architecture
• Existing ADRs: adr-001 through adr-003
• Template: decisions/adr-template.md