docs - adding more information on user installable commands and guild installable commands

[jacoobes]

In the modules section reference User Commands and Guild Commands. We should reference this to the Publisher Section since these are integrated pretty deeply.

• talk about user-installable commands really quickly
  • What are they?
  • A link to installing them through the discord dev portal
  • An example
• on the flipside,
  • what are guild installable commands?