refactor: restructure the compact lang ref

[0xisk]

PR Changes:

• Restructured the Compact language reference (lang-ref.mdx) into a hierarchical set of sub-pages. Each header 2 (##) is now a separate page, with its children (header 3 ### and header 4 ####) as nested sub-pages, matching the sidebar hierarchy.
  That makes it way easier to add the future edits like examples.

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Project	Deployment	Preview	Comments	Updated (UTC)
midnight-wiki	Ready	Preview	Comment	Nov 13, 2025 2:51pm

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