docs: make the BYOT guide provider-neutral and SAS-facing

[Hou (SciencePotato)]

What changed

Replace the merged provider checklist with a provider-neutral SAS/BYOT guide that explains the request and response, then shows customers where to implement their own integration.

The guide follows the request path: verify the caller, read and decrypt the payload, evaluate or send, and respond to SAS. Core fields remain in short tables; detailed runtime differences and settings are linked from the contract and setup references.

The new implementation section gives customers concrete repository locations:

• Choose one runtime and create the provider module or class in its provider directory.
• Implement the existing provider hooks and register the provider in the JavaScript selector, C# dependency injection, or Python provider dictionary.
• Use the existing credential/configuration services and local.settings.sample.json; settings select registered code but do not create an adapter.
• Add tests, package the changed source, and apply cloud settings and trust configuration through the documented deployment path.

Customers own outbound mapping, authentication, and acceptance/failure interpretation. The shared handler keeps SAS authentication boundaries, envelope validation, decryption, evaluation, and nonce responses. The guide names no phone provider and includes no vendor API recipes or inline implementation samples.

The existing guide path is retained, with navigation updated in the main, technical, and runtime guides. Closely related contract corrections reflect actual runtime binding and response differences without changing implementation behavior.

Review and verification

Three earlier passes checked flow, source accuracy, and natural wording. The implementation-map follow-up was reread as “which file do I open, what do I add, and what comes next?” Exact provider hooks, registration locations, config readers, credential services, and test directories were checked against source.

The latest follow-up passes 44 local link/anchor checks, including all five inbound references; preserves all 13 preceding guide anchors; and renders four tables correctly. Required fields and implementation hooks are covered, with no named providers, code fences, or images in the guide. git diff --check passes.

Seven Markdown files in the overall PR; the latest follow-up changes only the BYOT guide. No runtime, infrastructure, catalog, Azure, or provider actions. Runtime tests were not run for documentation-only changes.

Reviewed head: 01973a4. Open for review; no merge requested.