Synchronizes README.md Mermaid diagrams with current public API surface.

[jmprieur]

Synchronizes README.md Mermaid diagrams with current public API surface.

Changelog:

• Added Algorithm to CredentialDescription.
• Added AdditionalResponseParameters and BindingCertificate to AcquireTokenResult.
• Added Name property to MicrosoftEntraApplicationOptions.
• Corrected property name ExtraHeadersParameters -> ExtraHeaderParameters and fixed types to IDictionary<string,string>.
• Ensured ExtraQueryParameters / ExtraHeaderParameters displayed as dictionaries throughout.
• Marked HttpMethod in DownstreamApiOptionsReadOnlyHttpMethod as read-only.
• Added generic interface IAuthorizationHeaderProvider.
• Added condensed vs expanded IDownstreamApi diagrams.
• Added extensibility diagram (credential loaders & custom signed assertion providers).
• Annotated default values and audience union behavior.
• Updated serializer/deserializer signatures to show nullability.
• Added synchronization badge (date 2025-10-15).

Documentation-only change; no runtime behavior modifications.
Done with Copilot

[jmprieur]

Closing as it was removing too much of the content.
Follow-up in #220