Add IActivator<T> custom activators documentation for Orleans

[ReubenBond]

This is a work-in-progress. We had some discussions on the discord server this morning about gaps in the docs when it comes to advanced serialization scenarios and so this PR tries to address the gaps we identified. I used copilot to write the doc, grounded in the orleans codebase, and I need to refine it before it's ready to be merged.

Summary

Adds a new documentation page covering the IActivator<T> interface and [RegisterActivator] attribute in Orleans serialization. This feature allows developers to control how objects are constructed during deserialization.

Changes

• New page: docs/orleans/host/configuration-guide/custom-activators.md
• Buildable snippets: snippets/custom-activators/ with a .csproj and 4 .cs files
• TOC update: Nested under the existing Serialization entry

Content

The doc covers:

• What IActivator<T> is and when to use it
• The [UseActivator] and [RegisterActivator] attributes
• Buildable code examples for:
  • Basic custom activator
  • Object pooling with ObjectPool<T>
  • Dependency injection
  • Generic type activators
• How the activator pipeline works internally

All snippet files compile against Microsoft.Orleans.Sdk 9.0.0 (net9.0).

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Internal previews

📄 File	🔗 Preview link
docs/orleans/host/configuration-guide/custom-activators.md	Custom activators with IActivator<T>
docs/orleans/host/configuration-guide/serialization-lifecycle-hooks.md	Serialization lifecycle hooks
docs/orleans/toc.yml	docs/orleans/toc