Restructure TOC: Add groups, captions, and icons

[bmunkholm]

Summary of the changes / Why this is an improvement

Fixes parts of https://github.com/crate/tech-content/issues/169 together with crate/cratedb-guide#547

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch bm/toc-restructure

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