Docs subtask: update GeneralUpdate.Core component documentation

[JusterZhu]

Parent: #45

Scope

Document GeneralUpdate.Core for application update scenarios, excluding firmware content.

Acceptance criteria

• Explains Core responsibilities in the Client/Upgrade architecture.
• Covers GeneralUpdateBootstrap, events, errors, OSS mode, and server mode.
• Includes a minimal runnable example and links to Samples and Tools artifacts.

[JusterZhu]

Draft PR opened for the first GeneralUpdate.Core documentation draft: #56

[JusterZhu]

Draft PR opened for the first GeneralUpdate.Core documentation draft: #56

[JusterZhu]

Completed via PR #56: GeneralUpdate.Core documentation has been comprehensively rewritten with full API coverage, strategy overviews, and knowledge maps.