[WIP] fix: cover page button style

[cheng-kang]

Closes #665

Issue

CSS selector for cover page buttons are not working due to change of marked output. No <p> wrapper anymore.

Solution

Update CSS selector solves the problem.

Discussion

Although updating CSS selector solves the problem, it brings a potential issue: all direct child <a> tags of .cover-main will now be rendered as a "cover page button".

Another way of solving this might be adding a new wrapper to buttons, or add configs ([GitHub](https://github.com/docsify :cover-page-button)) to links to declare that they should be styled as "cover page buttons".

@QingWei-Li what's your opinion?

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[QingWei-Li]

We should parse markdown use #lexel.
This makes it easier and safer to render cover.

[QingWei-Li]

A temporary solution: find brother elements...

.cover-main
 &>blockquote~a,
 &>ul~a,
 &>h1~a
   ...