Document markdown behaviors

[wxiaoguang]

Feature Description

• Fix markdown render behaviors #34122
• Refactor markup render to fix various path problems #34114
• Support creating relative link to raw path in markdown #34105
• Markdown Syntax Guide #14297
• Refactor markdown math render, add dollor-backquote syntax support #32831

[wxiaoguang]

Done in https://gitea.com/gitea/docs/pulls/198/files