Keep Embedded feature guides specific to Laravel

[rmcdaniel]

Summary

• Keep concurrency, sagas, and cancel/terminate focused on embedded Laravel. Link service-mode readers to the existing PHP/Python/Rust SDK sites, where the examples and APIs are already published.
• Correct the embedded cancellation promise against WorkflowStub: both cancel and terminate immediately close the run; neither schedules cleanup afterward.
• Move Portable Worker Affinity to Service Mode, redirect both former URLs, preserve responsive table styling, and identify Python/Rust features as not supported.
• Add the placement convention to this repository's contributor instructions.

Refs #125; close after deployment verification. Actual SDK parity remains separate in durable-workflow/.github#124.

Validation

• Local Node 24 production build and LLM bundle generation passed.
• Rendered HTML: the three embedded articles have no Python/Rust code blocks; worker affinity appears under Service Mode only; both old routes point to the new page.
• Live SDK topic links and anchor checks passed. The PHP portal does not generate the inferred saga heading anchor, so the link targets its published workflows/activities page.
• Rust nested-parallel and saga source examples already return HTTP 200 on rust.durable-workflow.com; Python's published reference already contains its parallel/saga examples and explicit cancellation handling.
• GitHub checks run on the final commit before merge. No SDK runtime changes.