add more documentation for the RGB underglow

[nicolai86]

I've reorganized the RGB underglow documentation a little to avoid the issues I had:

• point out that it's easier to get RGB underglow to work if the left side is the master. I think there's a way to make it work on the right side too, but I've yet to get that to work reliably.
• add a list of required parts to support a detachable RGB underglow
• add images of completed setup: during soldering it's useful to check your own work against a documented working setup, which is now present
• add a link to the RGB underglow in the main documentation to point out it should be done before any switches are soldered.

[nicolai86]

Thank you @jpconstantineau !