fix($markdown): invalid toc links

[haoranpb]

Summary

fix #1985 , fix #2030

Credit goes to markdown-it-table-of-contents and #1375

Instead of slugify the headers directly, try to find the id created by markdown-it-anchor first

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• Bugfix
• Feature
• Code style update
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Other information:

[meteorlxy]

Great work 👍

[ulivz]

Could you add more tests, e.g. using Vue components at headers?