Documentation: new doc on Storage module and JS cleanup (#4161)

- Add new doc on Storage module
- Improve formatting in the JS section for better readability

documentation/js/js_gpio.md

@@ -21,21 +21,26 @@ led.write(false);
 delay(1000);
 ```
 
+---
+
 # API reference
-## `get`
+## get
 Gets a `Pin` object that can be used to manage a pin.
 
-### Parameters
+**Parameters**
   - `pin`: pin identifier (examples: `"pc3"`, `7`, `"pa6"`, `3`)
 
-### Returns
+**Returns**
+
 A `Pin` object.
 
-## `Pin` object
-### `Pin.init()`
+<br>
+
+## Pin object
+### Pin.init()
 Configures a pin.
 
-#### Parameters
+**Parameters**
   - `mode`: `Mode` object:
     - `direction` (required): either `"in"` or `"out"`
     - `outMode` (required for `direction: "out"`): either `"open_drain"` or
@@ -46,30 +51,41 @@ Configures a pin.
       `"rising"`, `"falling"` or `"both"`
     - `pull` (optional): either `"up"`, `"down"` or unset
 
-### `Pin.write()`
+<br>
+
+### Pin.write()
 Writes a digital value to a pin configured with `direction: "out"`.
 
-#### Parameters
+**Parameters**
   - `value`: boolean logic level to write
 
-### `Pin.read()`
+<br>
+
+### Pin.read()
 Reads a digital value from a pin configured with `direction: "in"` and any
 `inMode` except `"analog"`.
 
-#### Returns
-Boolean logic level
+**Returns**
 
-### `Pin.readAnalog()`
+Boolean logic level.
+
+<br>
+
+### Pin.readAnalog()
 Reads an analog voltage level in millivolts from a pin configured with
 `direction: "in"` and `inMode: "analog"`.
 
-#### Returns
+**Returns**
+
 Voltage on pin in millivolts.
 
-### `Pin.interrupt()`
+<br>
+
+### Pin.interrupt()
 Attaches an interrupt to a pin configured with `direction: "in"` and
 `inMode: "interrupt"` or `"event"`.
 
-#### Returns
+**Returns**
+
 An event loop `Contract` object that identifies the interrupt event source. The
 event does not produce any extra data.