docs: add emailConfig and per-request fromName/subject for Built-in IdP

[k1LoW]

Add documentation for the Built-in IdP email customization features: namespace-level email configuration (emailConfig) and per-request fromName/subject overrides for password reset emails.

Changes:

• Added emailConfig option (fromName, passwordResetSubject) to the defineIdp() configuration example and SDK reference in docs/sdk/services/idp.md.
• Added a new "Email Configuration" section in docs/guides/auth/integration/built-in-idp.md explaining the priority chain (per-request → namespace default → hardcoded default).
• Updated _SendPasswordResetEmailInput GraphQL documentation with optional fromName and subject fields.
• Updated SendPasswordResetEmailInput interface and examples in docs/guides/function/managing-idp-users.md with the new optional fields.
• Updated sendPasswordResetEmail example in docs/guides/function/builtin-interfaces.md to show the new parameters.