docs: restructure contribution guide sections

Major reorganization of the contribution guide:
- Rename contribution sections: 代码贡献, 文档贡献, 组件贡献
- Remove 提交blog and 网站贡献指南 sections (merged into 文档贡献)
- Merge 网站贡献指南 content into 文档贡献 section
- Add two contribution methods to 文档贡献: quick web edit and full Git workflow
- Add preview URL explanation after PR submission
- Remove 第二步：环境准备 from 组件贡献 section
- Rename steps from Chinese numerals to Arabic numerals for consistency
- Remove step numbering prefixes (第1步, 第2步, etc.) from headings
- Change compatibility label to extension label
- Simplify 第4步 title from 编辑文档或修改代码 to 修改代码
- Add document center structure explanation to 文档贡献

Author: jiaoshuntian