fix(theme-classic): make hash link in heading not selectable

[Jess-2021]

Motivation

Fix #5990. I added an attribute about 'user-select' to the hash-link class. And it can help me to copy the text without worrying about ZWSP.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Manual verification.

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	59
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-6000--docusaurus-2.netlify.app/

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[Josh-Cena]

Thanks, makes sense

Congrats on making issue #6000 :D