Document the bound domain in provider initialization and domain-scoped providers

The spec now supplies the bound domain to a provider's initialize function, and adds an opt-in domain-scoped provider declaration that the API enforces by binding such a provider to at most one domain. The conceptual docs should cover this so provider authors and users understand it.

What needs to happen

• Document that initialize can receive the bound domain, and when a provider would use it (e.g. scoping a persistent cache)
• Document the domain-scoped provider concept: what declaring it means, and that the API binds such a provider to at most one domain
• Fit this into the existing provider/domain and provider-lifecycle docs rather than a standalone page

Related

• #1387 (provider lifecycle guidance), #119 (init/shutdown docs)

Links

• Parent issue: https://github.com/open-feature/spec/issues/403
• Spec sections: 1.1 Flag evaluation, 2.4 Initialization

Start with the existing provider/domain and provider-lifecycle documentation referenced in the issue, using related issues #1387 and #119 for context. Document that initialize can receive the bound domain, when it is useful, and what declaring a domain-scoped provider means. Done when the API's one-domain binding rule and both provider concepts are covered in the existing documentation.