Document the new end-user opt-in flow

[roncohen]

Summary

• update the beta opt-in guide for the React SDK useOptInFlags() and useSetOptIn() flow
• show the { flags, isLoading } result and render loading before an empty state
• explain that bootstrapped providers fetch opt-in metadata on first use; regular providers use their normal loading state
• document provider-level and per-hook React Suspense behavior
• document flag setup, user and company scopes, access-mode behavior, and update synchronization
• remove the unlisted duplicate product handbook page

Documents the SDK API finalized by reflagcom/javascript#697.

Verification

• git diff --check
• verified the guide's internal Access rules link resolves