Add docs for developer Mode

[stevengill]

Description

We need to add a section to our docs (basic concept?) for the newly introduced Developer Mode (see #714, #742). Maybe this could also get added to a getting started guide for Socket mode?

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [ ] enhancement (feature request)
• [ ] question
• [x] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

[filmaj]

Good idea @stevengill !

Perhaps an additional section after the Socket Mode docs, perhaps in a similar vein, something like "Developer Mode"?

[stevengill]

Yeah, not a bad idea. Thing is, developer mode doesn't do much yet. It has additional logs (shows payloads of events being received that weren't handled with listeners), and it auto sets socketMode:true. I think that is all it does today. We should consider what else we could do with it.