refactor(v2): clean-ups and fixes

[lex111]

Motivation

Small minor fixes:

• Remove/optimize of redundant CSS properties.
• Remove non-working CSS class name
• Fix bug with hideable navbar (with multiple and fast scrolling at beginning of page, the navbar was incorrectly hidden)

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Preview

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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Size Change: +15 B (0%)

Total Size: 158 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	17.6 kB	0 B
website/build/assets/js/main.********.js	111 kB	+15 B (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	21.7 kB	0 B
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	6.95 kB	0 B

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	78
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

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