feat(gitcommit): enhance documentation and introduce Git tool features

- update extension description to include advanced Git operations via @git_bot
- add new section for Git Tool Operations detailing natural language commands
- revise configuration options to include @git_bot integration settings
- document new commands for interactive Git assistant in chat buffers
- expand features list to highlight comprehensive Git operations and safety features
- improve overall structure and clarity of documentation for better user guidance

Author: jinzhongjia