doc: Separate pages into sub directories.

docs/api/browser/power-monitor.md

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+# power-monitor
+
+The `power-monitor` module is used to monitor the power state change, you can
+only use it on the browser side.
+
+An example is:
+
+```javascript
+require('power-monitor').on('suspend', function() {
+  console.log('The system is going to sleep');
+});
+```
+
+## Event: suspend
+
+Emitted when the system is suspending.
+
+## Event: resume
+
+Emitted when system is resuming.