fix(v2): baseUrl help banner should not be indexed by Google / SEO

[slorber]

Motivation

#4028

The help banner that is visible when the site baseUrl is badly configured gets indexed by Google by mistake (despite being made invisible on sites correctly configured, thanks to critical CSS)

This PR changes the way this banner is rendered:

• The banner is not server-rendered anymore (not in static HTML files)
• The banner is rendered on the client, using critical inlined JS

This should theoretically ensure Google will not index that string (can't be 100% sure)

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Production

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[V1] Deploy preview success

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[github-actions]

Size Change: +4 B (0%)

Total Size: 28.8 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/blog/2017/12/14/introducing-docusaurus/index.html	21.8 kB	+3 B (0%)
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	6.87 kB	+1 B (0%)

_{compressed-size-action}

[netlify]

Deploy preview for docusaurus-2 ready!

Built with commit cba0f4c

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	87
🟢 Accessibility	99
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4125--docusaurus-2.netlify.app/classic/