[Fix] Fix lint issues in Markdown.

[keli-wen]

Description

The current markdown document has some basic formatting errors. Some of them have no impact, while others will affect the rendering effect. For example:

xxxx
- xx
- yy

should be modified to:

xxxx

- xx
- yy

to render correctly.

Based on this, I want to：

• use markdownlint to fix some obvious problems and
• add an automated markdown lint checker in the future.

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I found the issue when I read the #39.