Focus opt-in guide on setup and link to the SDK example

[roncohen]

Summary

• Remove the duplicated React implementation and detailed SDK API/loading guidance from the opt-in guide.
• Keep the guide focused on flag configuration, required context, user/company scope, and access behavior, with a direct link to the React SDK opt-in example.
• Update the handbook's description of the guide so it no longer promises an embedded step-by-step implementation.

The React SDK documentation is the single source of truth for the implementation example. No files under sdk/ are changed; they remain managed in the JavaScript repository and copied here.

Related SDK fixes: reflagcom/javascript#729

Validation

• Validated all 7 local links and heading anchors in the changed pages, including the SDK example link.
• Confirmed the guide contains no duplicated code examples.
• git diff --check passed.

Only guides/self-opt-in.md and the handbook's link description are changed. Unrelated local API docs and assets are excluded.

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