Keep embedded feature guides separate from service-mode SDK documentation

[rmcdaniel]

The Embedded feature guides currently mix Laravel workflow-package examples with Python/Rust service-mode guidance. This affects concurrency, sagas, and cancel/terminate, and makes the deployment boundary unclear.

Acceptance:

• Keep these three feature guides focused on embedded Laravel APIs and behavior.
• Preserve service-mode guidance on the relevant PHP/Python/Rust SDK documentation sites, using direct topic links instead of embedding SDK code in Embedded guides.
• Build the affected documentation, verify links, deploy, and check the published pages.

[rmcdaniel]

The worker-affinity page is also incorrectly under Embedded. Source inspection confirms that Python and Rust explicitly refuse all three capabilities (local activities, sessions, sticky execution), so this is both a placement defect and a real SDK parity gap. The docs fix moves the page to Service Mode, preserves the old URLs through redirects, and says Not supported without implying parity. SDK implementation is tracked separately in durable-workflow/.github#124.

[rmcdaniel]

Deployed and verified: PR #126, source commit fa79c73.

• Concurrency, sagas, and cancel/terminate return HTTP 200 with embedded Laravel scope and no Python/Rust code blocks.
• Existing SDK-site references/examples remain available; linked URLs and fragments were checked against the live sites.
• Portable Worker Affinity is under Service Mode only, with Python/Rust explicitly marked Not supported. Both former current and /docs/2.0/ paths redirect to the new page.
• The current embedded cancellation behavior is now documented accurately; no SDK/runtime behavior was changed.
• Production build, generated LLM bundles, all PR checks, and Pages deployment passed.

Deployment: https://github.com/durable-workflow/durable-workflow.github.io/actions/runs/34140749199
Live affinity reference: https://durable-workflow.com/docs/polyglot/portable-worker-affinity/

Documentation acceptance is complete. Actual Python/Rust worker-affinity implementation remains tracked in durable-workflow/.github#124 and is not closed by this fix.