New design for the widgets section in the docs (#866)

* Initial commit for the new widgets section

* Placing all the widgets and refining the CSS and Hugo logic

* Initial commit for the new widgets section

* Placing all the widgets and refining the CSS and Hugo logic

* Rebased and updated the info according to @verythorough contribution

* Fixing the yaml codes for the relation and select widget sections

* Merging with widgets.md and app.j

* Fixing some silly mistakes (sorry!)

* Following @verythorough contributions :)

* Adding the markdown widget and fixing the widgets container background

* Adding the URL functionality and myself as a contributor :)

* Adding myself as a contributor :)

website/site/content/docs/widgets/datetime.md

@@ -0,0 +1,25 @@
+---
+label: "DateTime"
+target: "datetime"
+type: "widget"
+---
+
+### DateTime
+
+The datetime widget translates a datetime picker to a datetime string. For saving the date only, use the date widget.
+
+- **Name:** `datetime`
+- **UI:** datetime picker
+- **Data type:** Moment.js-formatted datetime string
+- **Options:**
+  - `default`: accepts a datetime string, or an empty string to accept blank input; otherwise defaults to current datetime
+  - `format`: accepts Moment.js [tokens](https://momentjs.com/docs/#/parsing/string-format/); defaults to ISO8601 format `YYYY-MM-DDTHH:mm:ssZ`
+- **Example:**
+
+  ```yaml
+  - label: "Start time"
+    name: "start"
+    widget: "datetime"
+    default: ""
+    format: "LLL"
+  ```