Let Hugo's Node symlink check pass the public snapshot

[chalin]

• Scope:
  • Retires the env override the maintainers' snapshot build has needed since 0.18.0
  • Homes the snapshot layouts and their rationale in CONTRIBUTING § Built-site snapshot
• Out of scope:
  • Docsy's own docsy.dev/public symlink (its dev build runs no Node tool)
  • The Hugo upgrade post's wording for this symlink shape (guide feedback, routed separately)
  • _commit:public committing the surrounding repo when public is a plain directory (pre-existing; follow-up)
• Validation, with HUGO_SECURITY_* unset:
  • Builds pass at the primary clone through the symlink and in a worktree with a symlink to the snapshot; each fails on Hugo's symlink check when its entry is dropped
  • Builds pass in a worktree with a plain public/, and in CI, where neither snapshot path exists

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