Add Developer Documentation for BIP39-Based DID in Rubix Network

[gklps]

This PR adds a comprehensive developer guide titled "BIP39-Based Decentralized Identity (DID) in Rubix Network" in Markdown format.

The document provides:

A conceptual overview of DIDs and their importance

Detailed explanation of Rubix’s BIP39-based DID mechanism

Step-by-step instructions to create, register, and recover DIDs

Emphasis on verifiable claims, privacy, and trust

Code snippets and terminal commands for developer onboarding

Standards compliance information (W3C, GDPR, CCPA)