refactor(theme-classic): DocPage theme refactors polish

[slorber]

Motivation

DocPage changes:

• extract useDocRouteMetadata technical code to theme-common
• Rename Aside to Sidebar component
• Extract ExpandButton as a separate theme component
• properly split CSS files (useful for swizzle)

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

preview works exactly as before

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	a7131ee
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/626c142fdff215000899ac77
😎 Deploy Preview	https://deploy-preview-7268--docusaurus-2.netlify.app
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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	60
🟢 Accessibility	100
🟠 Best practices	83
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7268--docusaurus-2.netlify.app/

[github-actions]

Size Change: +217 B (0%)

Total Size: 806 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	107 kB	-73 B (0%)
website/build/assets/js/main.********.js	609 kB	+290 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50.7 kB
website/build/index.html	38.8 kB

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