flake.lock: Update

Flake lock file updates:

• Updated input 'mmdoc':
    'github:ryantm/mmdoc/cec02bafac9456bd1ed9b261b8d163a893885e5b' (2022-11-21)
  → 'github:ryantm/mmdoc/b6ddf748b1d1c01ca582bb1b3dafd6bc3a4c83a6' (2024-03-17)
• Removed input 'mmdoc/nixpkgs-for-manual'
• Added input 'mmdoc/systems':
    'github:nix-systems/default/da67096a3b9bf56a91d16901293e51ba5b49a27e' (2023-04-09)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/293a28df6d7ff3dec1e61e37cc4ee6e6c0fb0847' (2022-12-30)
  → 'github:NixOS/nixpkgs/807c549feabce7eddbf259dbdcec9e0600a0660d' (2024-03-29)