Add Maven dependency management documentation

[ivanauth]

This PR adds documentation to help Maven users resolve dependency convergence issues when using authzed-java with the maven-enforcer-plugin.

What's included

README.md

• New section on Maven dependency convergence with a practical example using the gRPC BOM
• Updated installation examples to reference the current versions (1.5.4 and gRPC 1.75.0)

MAVEN_BEST_PRACTICES.md (new file)

• Comprehensive guide explaining dependency convergence and why it matters
• Three approaches ranked by preference: gRPC BOM (recommended), explicit version pinning, and exclusions
• Complete working example with enforcer plugin configuration
• Troubleshooting section for common issues
• Links to official Maven and gRPC documentation

Why this matters

After the dependency convergence fixes in #152 and #153, users may still encounter convergence errors depending on their Maven setup. This documentation provides clear guidance on how to configure projects correctly.