docs: migrate // comments to /// rustdoc across all 6 contract crates

[chazepay]

• Replace floating // block comments at module level with //! crate-level docs
• Add structured /// rustdoc (# Arguments, # Returns, # Panics, # Examples) to
  every public fn, struct, enum variant, event struct, and const in all 6 crates:
  badge-nft, course-registry, reward-pool, stake-vault, quest-engine, governance
• Add 5+ doctests in /// blocks across badge-nft, course-registry, stake-vault,
  reward-pool, and governance
• Remove inline // step-comment scaffolding from all function bodies
• Add storage-layout tables to all DataKey and struct type docs
• Add cargo doc --no-deps --document-private-items zero-warning check to CI
• Add docs publish job to GitHub Pages on version tags (v*)

Closes #23

[merlik787-droi]

@chazepay resolve conflicts