docs(admin): add NatSpec for Role enum

[codenerde]

Summary

Adds comprehensive NatSpec documentation to the Role enum in contracts/admin/src/lib.rs.

Changes

• Added detailed enum-level documentation with:
  • Summary of the access-control model
  • Variant table with privilege levels and typical use cases
  • Storage invariant warning about variant ordering
  • Cross-references to related functions
• Enhanced individual variant docs with:
  • Admin: Documents implicit all-role grant and admin-only revoke/grant
  • Minter: Clarifies minting scope and what it doesn't imply
  • SuperAdmin: Explains reserved use cases and security recommendations
  • Pauser: Describes pause semantics and incident response context

Testing

• cargo fmt --check passes (0 issues)
• cargo clippy -- -D warnings passes (0 warnings)

Closes #489

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[p3ris0n]

@codenerde fix the conflict please