Add comprehensive Monarch Meshes documentation

[AlirezaShamsoshoara]

Summary

This PR introduces a complete guide to Monarch Meshes, the organizational backbone for managing distributed systems in Monarch. The documentation covers:

• Core Mesh Concepts: Detailed explanation of what meshes are, including their multidimensional structure, named dimensions, extents, and hierarchical organization
• Three Mesh Types: In-depth coverage of HostMesh (compute hosts/machines), ProcMesh (OS processes), and ActorMesh (actor instances), including their characteristics, relationships, and usage patterns
• Mesh Hierarchy: Visual diagrams and examples showing how meshes spawn other meshes, dimension inheritance, and the complete spawning flow from hosts to processes to actors
• Mesh Operations: Comprehensive guide to slicing (single index, range, and multi-dimensional), broadcasting, point-to-point selection, and extent manipulation
• Distributed Patterns: Four common distributed computing patterns with code examples and diagrams:
  • Simple Data Parallel
  • Parameter Server
  • Pipeline Parallel
  • Hierarchical Communication
• Best Practices: Production-ready guidance on mesh design, slicing patterns, dimension naming, extent validation, resource management, and testing strategies
• Visual Aids: 15+ Mermaid diagrams illustrating mesh structures, hierarchies, operations, and communication patterns

This comprehensive guide serves as the definitive resource for understanding and implementing mesh-based distributed computing in Monarch, complementing the existing actor and system overview documentation.

[AlirezaShamsoshoara]

cc @johnwhumphreys @colin2328 Adding you here since I cannot add reviewer directly