fix(theme-classic): restore docusaurus search meta

[slorber]

Motivation

In #6707 we replaced docusaurus_x meta by Algolia meta (biasing toward Algolia)

Unfortunately the former metas were actually used by some third-party search plugins: cmfcmf/docusaurus-search-local#99

To avoid annoying anyone for now I'm duplicating the metadata

In the future I'd like to avoid doing so (as it slightly increase page size uselessly), but we'll figure this later

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

preview

[netlify]

✔️ [V2]

🔨 Explore the source changes: 823eb0f

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6220d2a9c24d840008a2c82a

😎 Browse the preview: https://deploy-preview-6827--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	54
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-6827--docusaurus-2.netlify.app/

[github-actions]

Size Change: +310 B (0%)

Total Size: 790 kB

Filename	Size	Change
website/build/assets/js/main.********.js	596 kB	+190 B (0%)
website/build/index.html	38.4 kB	+120 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	49.7 kB
website/build/assets/css/styles.********.css	105 kB

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