feat(netlify-cms-widget-map): add map widget (#2051)

website/content/docs/widgets/map.md

@@ -0,0 +1,18 @@
+---
+title: map
+label: "Map"
+---
+
+The map widget allows you to edit spatial data using an interactive map. Spatial data is saved as a GeoJSON string in WGS84 projection and is limited to a single geometry.
+
+- **Name:** `map`
+- **UI:** interactive map
+- **Data type:** GeoJSON string
+- **Options:**
+  - `decimals`: accepts a number to specify precision of saved coordinates; defaults to 7 decimals
+  - `default`: accepts a GeoJSON string containing a single geometry; defaults to an empty string
+  - `type`: accepts one string value of `Point`, `LineString` or `Polygon`; defaults to `Point`
+- **Example:**
+    ```yaml
+    - {label: "Location", name: "location", widget: "map" }
+    ```