Add hybrid slash command variants to some public bot commands

[DragonSenseiGuy]

This issue proposes adding slash command alternatives (hybrid commands in discord.py) to some bot commands.

Prioritisation should be as follows:

• Commands that will have a better UX with slash commands (mostly things that would support autocomplete)
• Other public commands for which a slash command variant does not directly change or improve UX, but implementation is trivial
• Finally, Moderator+ and Helper+ commands can be considered.

[!NOTE]

There are commands that will never be migrated or have a slash command alternative as Discord provides no reasonable way to implement their input functionality. For example, the !eval command will never have an alternate slash command.

Hybrid command additions should be contributed in reasonable chunks (not all in one PR and similarly not a slurry of small PRs). Migrating one cog at a time seems a reasonable balance for both contributors and reviewers.

Some suggested first migration targets are the following, Core Devs please feel free to update this list to add further commands:

• [ ] !docs

[DragonSenseiGuy]

I think making the !help command into slash first, will be the best? As new users are more likely to use that.