feat(theme-classic): add Serbian Cyrillic translation

[utajum]

Motivation

Hacktoberfest 😳
Also #3526

Have you read the Contributing Guidelines on pull requests?

Yes

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[netlify]

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🔨 Explore the source changes: a155bf0

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	93
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5791--docusaurus-2.netlify.app/

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[lex111]

@utajum thanks!