docs: natspec comments + docgen

[rymnc]

Uses solidity-docgen to generate docs in the pre-commit hook

[github-actions[bot]]

LCOV of commit 71ba533 during CI #10

Summary coverage rate:
  lines......: 100.0% (492 of 492 lines)
  functions..: 100.0% (9 of 9 functions)
  branches...: 100.0% (24 of 24 branches)

Files changed coverage rate:
                              |Lines       |Functions  |Branches    
  Filename                    |Rate     Num|Rate    Num|Rate     Num
  ==================================================================
  contracts/PoseidonHasher.sol| 100%    461| 100%     2|    -      0
  contracts/Rln.sol           | 100%     31| 100%     7| 100%     24