fix: clarify slash command help and toggle behavior

[Xerrion]

Summary

• Makes DragonLoot slash commands clearer by reserving /dl for help and using an explicit toggle command.

Changes

• Updates /dl to show the help list instead of toggling the addon.
• Adds /dl toggle as the explicit command for enabling or disabling DragonLoot.
• Removes /dl enable and /dl disable from the help flow and command handling.
• Refreshes the README command section to match the current slash command behavior.

Testing

• GitHub Actions: luacheck