docs: document experimental Switchyard integration

[bbednarski9]

Overview

Add the public Fern documentation for Relay's experimental Switchyard Decision API plugin. This keeps the functional integration in NeMo Relay #369 separate from its user-facing documentation.

This PR makes the Switchyard configuration page the authoritative user-facing reference for the complete component contract, deployment limitations, and current native-integration boundary.

Experimental: The integration is early access, disabled in default Relay builds, and currently requires a separately running Switchyard service.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

• Add Configure Plugins → Switchyard (Experimental) to the Fern navigation.
• Organize configuration around the Decision API connection, profile selection, exact Relay-owned backend bindings, and trusted same-protocol defaults.
• Document every component option, enum, default, and validation constraint, including all six request-materialization modes and the requirement for all three protocol defaults.
• Provide complete configuration and recommended recipes for observe-only, payload-only request-aware, and ATOF-backed StageRouter routing.
• Explain fail-open behavior, retry boundaries, streaming commitment, exact target validation, and environment-referenced credential handling.
• Consolidate the experimental deployment limitations previously spread across crate and example documentation.
• Clarify that routing decisions and ATOF accumulation use a separately running Switchyard service while provider-protocol translation runs in process through switchyard-translation.
• Clarify the native-integration boundary: the Rust component is registered by Relay's CLI gateway; Hermes native plugins, LangChain, LangGraph, and Deep Agents receive Switchyard routing only when their provider traffic traverses that gateway.
• Document the pinned Switchyard topic-branch compatibility commit and identify the examples as manual compatibility and trajectory workflows rather than production orchestration.
• Keep all examples public-safe, environment-referenced, and free of internal-only provider assumptions.

The docs do not change Relay runtime behavior, configuration schemas, generated API surfaces, or PR #369.

Where should the reviewer start?

1. docs/configure-plugins/switchyard/configuration.mdx — complete option reference, recipes, runtime behavior, and experimental limitations.
2. docs/configure-plugins/switchyard/about.mdx — experimental status, ownership boundary, prerequisites, and direct links to the reference and limitations.
3. docs/index.yml — Fern navigation placement.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to NVIDIA/NeMo-Relay #369.
• Relates to NVIDIA-NeMo/Switchyard #46.

Validation

• npx fern check --warnings passed with zero errors; the unauthenticated redirects check was skipped with one warning.
• npx fern docs broken-links --strict found six pre-existing broken API-reference links in docs/resources/support-and-faqs.mdx and docs/reference/api/index.mdx; no errors point to the Switchyard pages.
• git diff --check passed.
• A content audit found no InferenceHub references, internal-only endpoints, literal credentials, local Visor paths, or unpublished test artifacts in the Switchyard Fern pages.

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