Add comprehensive documentation for GeneralUpdate.Extension plugin management system (#19)

* Initial plan

* Add GeneralUpdate.Extension documentation in Chinese and English

Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

* Fix HTML tag escaping in GeneralUpdate.Extension documentation

Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

* Complete rewrite: GeneralUpdate.Extension documentation for actual extension management system

Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

* Remove backup file

Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

Author: Copilot