Add comprehensive Actors documentation guide

[AlirezaShamsoshoara]

PR Summary

This PR adds a complete guide to Monarch actors (docs/ACTORS.md) that serves as the reference for understanding and implementing the actor model in Monarch applications.

The documentation covers:

• Actor fundamentals: Definition, core characteristics (isolation, sequential processing, asynchronous communication, location transparency)
• Actor lifecycle: Complete walkthrough from creation through termination, including spawn, construction, initialization, running, and termination phases
• Endpoints and messaging: Detailed explanation of the @endpoint decorator and messaging patterns including call_one(), call(), broadcast(), rref(), and stream()
• Actor context: How to access runtime information via the context API including message rank, actor instance details, and process references
• ActorMesh: Organization of actors in multidimensional structures with slicing, indexing, and broadcast operations
• Advanced patterns: Explicit response ports, actor supervision, channels for direct communication, and ValueMesh for state sharing
• Best practices: Actor design principles, state management, endpoint design, error handling, performance optimization, and testing strategies

The guide includes code examples, diagrams for visualization, and a quick reference card for common patterns. This documentation complements the existing Meshes guide and provides developers with everything needed to build robust actor-based distributed systems in Monarch.

[AlirezaShamsoshoara]

-- Adding @colin2328 @johnwhumphreys and @dulinriley since I cannot add reviewer!

[colin2328]

@AlirezaShamsoshoara can you import to fbsource?

[meta-codesync]

@AlirezaShamsoshoara has imported this pull request. If you are a Meta employee, you can view this in D87400422.

[meta-codesync]

@AlirezaShamsoshoara merged this pull request in b57ad38.