Removed "label" properties which no longer have any function.
Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.
* Document the Seeed(uino) XIAO interconnect
* Add metadata files for two upstream boards, XIAO and XIAO BLE.
* Add conf and overlay files to properly configure the boards
for ZMK use.