RBTools 1.0.3 is introducing the RBTOOLS_EDITOR environment variable,

for customizing the editing experience. This is one of a few environment

variables we offer, none of which are documented anywhere.
This change introduces a section to the User Configuration page to list

available environment variables. Any :envvar: references to these will

link to the correct place.

Built the docs. Checked for build and spelling errors.
Checked that each of the listed environment variables rendered correctly

and could be linked to.