Website fixes: data collections, pagination, write-portal preview & paste, UX fixes

[HumbleBee14]

• Topics and external tools moved out of code into content collections (src/content/topics, src/content/external-tools); post frontmatter still validated against topic files at build
• Blog and topic pages paginated (20/page) with numbered pager component
• Write portal: true article preview toggle, paste-screenshot support, upload image compression, pre-publish SEO suggestions, continue-writing affordance, quote block preview parity
• Fixes: playground scroll trap (wheel hand-off at container edges), related-card hover outline clipping, tables centered in articles and editor
• Contribute page leads with the /write editor; manual MDX demoted to a side note

[cloudflare-workers-and-pages]

Deploying mlsystems with    Cloudflare Pages

Latest commit:	e2ed63c
Status:	⚡️  Build in progress...

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