docs: add superscript and subscript description

src/resources/docs/markdown_guide_en.md

@@ -202,6 +202,16 @@ This is a footnote [^1].
 [^1]: Here is the detail of the footnote.
 ```
 
+### Superscript and Subscript
+
+> You need to enable Superscript or Subscript in the `Markdown` menu and use `Markdown-it` renderer.
+
+```md
+This is the 1^st^ superscript.
+
+This is the H~2~O subscript.
+```
+
 ### New Line and Paragraph
 If you want to enter a new line, you should add two spaces after current line and then continue your input. VNote provides `Shift+Enter` to help.
 

src/resources/docs/markdown_guide_zh.md

@@ -203,6 +203,16 @@ This is a footnote [^1].
 [^1]: Here is the detail of the footnote.
 ```
 
+### 上标和下标
+
+> 需要在`Markdown`菜单中启用上标和下标并使用`Markdown-it`渲染引擎。
+
+```md
+This is the 1^st^ superscript.
+
+This is the H~2~O subscript.
+```
+
 ### 换行和段落
 如果需要换行，您应该在当前行末尾添加两个空格，然后换行。VNote提供快捷键`Shift+Enter`来辅助用户输入两个空格并换行。