.editorconfig: set docbook indent size

[zowoq]

#87853 (comment)
DocBook is actually indented with a single space (default value for xmlformat).

[zowoq]

Added gemfiles and json, they seem to be the most common that aren't already listed here.

Dropped them as some are unusual and they aren't that common to be worth worrying about at this stage.

[zowoq]

json looks fine for most things except for node-packages/*.json (which I should have checked first), guess that's probably a dealbreaker. I'll drop it from here if it is.