Brick ref topics 02 (#283)

* Add brick button topics

* Add the rest of the brick api

docs/reference/brick/battery-level.md

@@ -0,0 +1,33 @@
+# battery Level
+
+Return the current battery level.
+
+```sig
+brick.batteryLevel();
+```
+
+## Returns
+
+* a [number](/types/number) which is the current charge level of the brick's battery. This is a percentage of total charge left in the battery.
+
+## Example
+
+Show the battery level percentage on the screen. Also, show a green light if the battery level is above 15%. If the battery level is below 15% but above 5%, show a orange light. But, if the battery level is below 5%, show a pulsing red light.
+
+```blocks
+let battery = 0;
+loops.forever(function() {
+    brick.showString("Battery level:", 1)
+    brick.showNumber(battery, 2)
+    battery = brick.batteryLevel();
+    if (battery > 15)
+    {
+        brick.setLight(BrickLight.Green);
+    } else if (battery > 5) {
+        brick.setLight(BrickLight.Orange);
+    } else {
+        brick.setLight(BrickLight.RedPulse)
+    }
+    loops.pause(30000)
+})
+```