[Rules]: use @commands.guild_only() decorator.

[Evanroby]

Used @commands.guild_only() decorator so it doesn't give an error when running in dms:

Traceback (most recent call last):
  File "{HOME}/redenv/lib/python3.11/site-packages/discord/ext/commands/core.py", line 235, in wrapped
    ret = await coro(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{HOME}/.local/share/Red-DiscordBot/data/axion/cogs/CogManager/cogs/rules/rules.py", line 25, in send_rules
    rules_channel_id = await self.config.guild(ctx.guild).rules_channel_id()
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{HOME}/redenv/lib/python3.11/site-packages/redbot/core/config.py", line 982, in guild
    return self._get_base_group(self.GUILD, str(guild.id))
                                                ^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'id'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "{HOME}/redenv/lib/python3.11/site-packages/discord/ext/commands/bot.py", line 1366, in invoke
    await ctx.command.invoke(ctx)
  File "{HOME}/redenv/lib/python3.11/site-packages/redbot/core/commands/commands.py", line 825, in invoke
    await super().invoke(ctx)
  File "{HOME}/redenv/lib/python3.11/site-packages/discord/ext/commands/core.py", line 1650, in invoke
    await ctx.invoked_subcommand.invoke(ctx)
  File "{HOME}/redenv/lib/python3.11/site-packages/discord/ext/commands/core.py", line 1029, in invoke
    await injected(*ctx.args, **ctx.kwargs)  # type: ignore
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{HOME}/redenv/lib/python3.11/site-packages/discord/ext/commands/core.py", line 244, in wrapped
    raise CommandInvokeError(exc) from exc
discord.ext.commands.errors.CommandInvokeError: Command raised an exception: AttributeError: 'NoneType' object has no attribute 'id'

Also did Black reformatting, and formatted imports.