feat(docs): Update power and lighting config pages

Moved battery configuration to its own page to match the feature page.

Documented that external power is disabled when in sleep mode.

Clarified that the *_START configs apply on first boot, and any changes
after that are persisted.

docs/sidebars.js

@@ -55,6 +55,7 @@ module.exports = {
     Configuration: [
       "config/index",
       "config/backlight",
+      "config/battery",
       "config/behaviors",
       "config/combos",
       "config/displays",
@@ -62,8 +63,8 @@ module.exports = {
       "config/keymap",
       "config/kscan",
       "config/power",
-      "config/system",
       "config/underglow",
+      "config/system",
     ],
     Development: [
       "development/clean-room",