Splitting documentation into several pages. Changing documentation to adapt to new "namespaces"

docs/maps/api-ui.md

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+{.section-title.accent.text-primary}
+# API Room functions Reference
+
+### Opening a popup
+
+In order to open a popup window, you must first define the position of the popup on your map.
+
+You can position this popup by using a "rectangle" object in Tiled that you will place on an "object" layer.
+
+<div class="row">
+    <div class="col">
+        <img src="https://workadventu.re/img/docs/screen_popup_tiled.png" class="figure-img img-fluid rounded" alt="" />
+    </div>
+    <div class="col">
+        <img src="https://workadventu.re/img/docs/screen_popup_in_game.png" class="figure-img img-fluid rounded" alt="" />
+    </div>
+</div>
+
+```
+WA.ui.openPopup(targetObject: string, message: string, buttons: ButtonDescriptor[]): Popup
+```
+
+*   **targetObject**: the name of the rectangle object defined in Tiled.
+*   **message**: the message to display in the popup.
+*   **buttons**: an array of action buttons defined underneath the popup.
+
+Action buttons are `ButtonDescriptor` objects containing these properties.
+
+*   **label (_string_)**: The label of the button.
+*   **className (_string_)**: The visual type of the button. Can be one of "normal", "primary", "success", "warning", "error", "disabled".
+*   **callback (_(popup: Popup)=>void_)**: Callback called when the button is pressed.
+
+Please note that `openPopup` returns an object of the `Popup` class. Also, the callback called when a button is clicked is passed a `Popup` object.
+
+The `Popup` class that represents an open popup contains a single method: `close()`. This will obviously close the popup when called.
+
+```javascript
+class Popup {
+    /**
+     * Closes the popup
+     */
+    close() {};
+}
+```
+
+Example:
+
+```javascript
+let helloWorldPopup;
+
+// Open the popup when we enter a given zone
+helloWorldPopup = WA.room.onEnterZone('myZone', () => {
+    WA.ui.openPopup("popupRectangle", 'Hello world!', [{
+        label: "Close",
+        className: "primary",
+        callback: (popup) => {
+            // Close the popup when the "Close" button is pressed.
+            popup.close();
+        }
+    });
+}]);
+
+// Close the popup when we leave the zone.
+WA.room.onLeaveZone('myZone', () => {
+    helloWorldPopup.close();
+});
+```