feat: Add soft on/off support.

Initial work on a soft on/off support for ZMK. Triggering soft off
puts the device into deep sleep with only a specific GPIO pin
configured to wake the device, avoiding waking from other key
presses in the matrix like the normal deep sleep.

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

docs/sidebars.js

@@ -19,6 +19,7 @@ module.exports = {
       "features/underglow",
       "features/backlight",
       "features/battery",
+      "features/soft-off",
       "features/beta-testing",
     ],
     Behaviors: [
@@ -44,6 +45,7 @@ module.exports = {
       "behaviors/underglow",
       "behaviors/backlight",
       "behaviors/power",
+      "behaviors/soft-off",
     ],
     Codes: [
       "codes/index",