add via for cnc reference and replace mount pads

Furthermore bump version to v2.3 because we already etched v2.2 and
changed the board afterwards so e.g. milling some v2.2 is not possible
with current master anymore. There's still no tag for v2.2 but I guess
there just won't be any, maybe a branch must be enough for now.
move assemble sheets to new doc folder
neues aufsatzboard für USB, nicht fertig
Merge pull request #19 from LeSpocky/v2.3

V2.3