update website guidelines and block / as a prefix for message-based commands

[TheMoksej]

Is your feature request related to a problem? Please describe.
People shouldn't be able to submit bots with / as a prefix for message-based commands if they don't have slash commands

Describe the solution you'd like
Block / as a prefix when only Bot scope is selected.

Describe alternatives you've considered
None

Additional context
Edit the website guidelines and add a point regarding the / prefix and how we don't allow the / prefix for message-based commands if they don't have slash commands

This would save peoples time when submitting a bot, i.e. they wouldn't need to wait until we open a ticket and let them know about this rule.