refactor(docs): Move combos under Features.

* Since combos aren't a behavior, but a new high level keymap
  feature, move under Features section.

docs/sidebars.js

@@ -10,6 +10,7 @@ module.exports = {
     ],
     Features: [
       "features/keymaps",
+      "features/combos",
       "features/displays",
       "features/encoders",
       "features/underglow",
@@ -20,7 +21,6 @@ module.exports = {
       "behaviors/misc",
       "behaviors/hold-tap",
       "behaviors/mod-tap",
-      "behaviors/combos",
       "behaviors/reset",
       "behaviors/bluetooth",
       "behaviors/outputs",