docs: expand Orleans architecture documentation

[ReubenBond]

Expands the Orleans 10 implementation track into an advanced, source-backed architecture guide. It documents runtime topology, activation migration, membership and directory protocols, scheduling and messaging semantics, resource-optimized placement, serialization, persistent streams, provider authoring, and TestingHost internals while correcting stale defaults and APIs.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR expands the Orleans implementation documentation into an Orleans 10, source-linked architecture track aimed at runtime contributors and advanced users, with new deep-dive pages and substantial rewrites of existing conceptual docs.

Changes:

• Replaces several high-level implementation pages with Orleans 10-focused architecture explanations (runtime, scheduling, messaging semantics, placement/balancing, membership, grain directory, lifecycle, streams).
• Adds new architecture pages for runtime flow, activation lifecycle/migration, serialization/codegen internals, and provider authoring patterns.
• Updates the implementation track landing page to reflect Orleans 10 defaults, invariants, and cross-links between the new pages.

Show a summary per file

File	Description
docs/site/src/content/docs/implementation/testing.md	Reworks testing content into TestingHost internals and fidelity boundaries.
docs/site/src/content/docs/implementation/streams-implementation/index.md	Reframes stream internals around persistent pulling architecture and invariants.
docs/site/src/content/docs/implementation/streams-implementation/azure-queue-streams.md	Updates Azure Queue adapter documentation with current APIs and acknowledgement semantics.
docs/site/src/content/docs/implementation/serialization.md	Adds a new deep-dive on serialization/codegen, manifests, and extension points.
docs/site/src/content/docs/implementation/scheduler.md	Rewrites scheduling doc around turns, WorkItemGroup, and admission vs execution.
docs/site/src/content/docs/implementation/runtime-architecture.md	Adds a new end-to-end runtime call-path architecture page.
docs/site/src/content/docs/implementation/provider-authoring.md	Adds a new architecture guide for provider authoring patterns and validation/lifecycle.
docs/site/src/content/docs/implementation/orleans-lifecycle.md	Refocuses lifecycle documentation on the protocol and ordering invariants.
docs/site/src/content/docs/implementation/messaging-delivery-guarantees.md	Expands messaging semantics, routing repair vs retries, and timeout ambiguity.
docs/site/src/content/docs/implementation/load-balancing.md	Updates placement documentation to Orleans 10 defaults and opt-in rebalancing/repartitioning.
docs/site/src/content/docs/implementation/index.md	Rebuilds the implementation landing page as an Orleans 10 architecture track index.
docs/site/src/content/docs/implementation/grain-directory.md	Updates grain directory documentation to distinguish default vs experimental directories.
docs/site/src/content/docs/implementation/cluster-management.md	Replaces cluster management page with a membership protocol-focused architecture doc.
docs/site/src/content/docs/implementation/activation-lifecycle.md	Adds a new activation lifecycle + migration architecture page.

Copilot's findings

• Files reviewed: 14/14 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 14/14 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 14/14 changed files
• Comments generated: 0 new