Create a plugin dev guide for using the granular Access Control system

[cp2004]

It has been on my mind for a while that there is no guide on how to use this system for plugin authors, and the documentation rewrite seems like a good time for this to be written.

The issue OctoPrint/OctoPrint#4632 highlighted the need for some documentation here. The hook is documented, but I don't think some of the other stuff is.

I was thinking of:

• Making sure octoprint.access is documented properly in 'Internal modules'
• Adding a quick guide on how the permissions system works and why you should implement it in your plugin

If anyone else wants to pick this up, go for it! otherwise I will probably get to it at some point when more of the documentation is migrated across.