[ZEPPELIN-6111] Update to include optional classic UI instructions

### What is this PR for?
We've recently switched the default UI to the new version.

As a result, it's important to provide instructions for users who want to access the previous default UI.

This is a minimal guide for now, and more detailed documentation should be added in the future.


### What type of PR is it?
Documentation

### Todos
* [ ] - Task

### What is the Jira issue?
* Open an issue on Jira https://issues.apache.org/jira/browse/ZEPPELIN/6111

### How should this be tested?
* Strongly recommended: add automated unit tests for any new or changed behavior
* Outline any manual steps to test the PR here.

### Screenshots (if appropriate)

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No


Closes #4847 from tbonelee/add-guidance-to-classic-ui.

Signed-off-by: Cheng Pan <[email protected]>

Author: tbonelee

docs/assets/themes/zeppelin/img/ui-img/switch_to_classic_ui.png

@@ -19,10 +19,22 @@ limitations under the License.
 -->
 {% include JB/setup %}
 
-# Explore Apache Zeppelin UI
+# Explore Apache Zeppelin UI (Classic UI)
 
 <div id="toc"></div>
 
+## How to Enable and Switch to the Classic UI
+
+Starting from Zeppelin 0.12.0, this UI has become optional. To use this UI, please build with the following profile:
+
+```
+-Pweb-classic
+```
+
+Afterward, you can switch to the classic UI via the `Swtich to Classic UI` button in the settings menu of the new UI app.
+
+<img src="{{BASE_PATH}}/assets/themes/zeppelin/img/ui-img/switch_to_classic_ui.png" width="130" />
+
 ## Main home
 
 The first time you connect to Zeppelin ([default installations start on http://localhost:8080](http://localhost:8080/)), you'll land at the main page similar to the below screen capture.