Fix 1d20

Summary by CodeRabbit

• New Features

  • Default /roll now rolls 1d20.
  • Supports 1–10 dice and 2–100 sides (was min 2 dice, 6 sides).
  • Consistent validation and error messages for slash and mention-based “roll XdY”.
  • Shows individual roll results and totals.

• Documentation

  • Updated README with clearer intro, Usage examples (/roll, quant/sides parameters, mention-based rolling), and a Features section.
  • Revised configuration examples to reflect limits (max 10 dice, max 100 sides).