Ignore some automatic application-specific files.

Those aren't part of configuration itself; and should not be edited. And
probably will differ between instalations
Added edts as submodule.

It is somewhat problematic.  Since edts creates it's own catalog with
internal data, i can not add plugin on top-level.  Hence new directory.
Not tracking internal edts plugin files.
Changed edts load place.
Updated edts-erlang documentation root.

This is somewhat upseting that this needed to be done by hand.