[KYUUBI #5118][DOC] Add Flink KDF doc

### _Why are the changes needed?_
Add Flink KDF doc to close the issue #5118

<img width="1094" alt="image" src="https://github.com/apache/kyuubi/assets/3898450/23cd886e-c94a-406b-81a2-eb2caf4c1775">

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [ ] [Run test](https://kyuubi.readthedocs.io/en/master/contributing/code/testing.html#running-tests) locally before make a pull request

### _Was this patch authored or co-authored using generative AI tooling?_
No

Closes #5233 from zhaohehuhu/Improvement-0901.

Closes #5118

fcd45b3 [hezhao2] code reformat
46bb970 [hezhao2] Add Flink KDF doc

Authored-by: hezhao2 <[email protected]>
Signed-off-by: Shaoyun Chen <[email protected]>

docs/extensions/engines/flink/functions.md

@@ -0,0 +1,30 @@
+<!--
+- Licensed to the Apache Software Foundation (ASF) under one or more
+- contributor license agreements.  See the NOTICE file distributed with
+- this work for additional information regarding copyright ownership.
+- The ASF licenses this file to You under the Apache License, Version 2.0
+- (the "License"); you may not use this file except in compliance with
+- the License.  You may obtain a copy of the License at
+-
+-   http://www.apache.org/licenses/LICENSE-2.0
+-
+- Unless required by applicable law or agreed to in writing, software
+- distributed under the License is distributed on an "AS IS" BASIS,
+- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+- See the License for the specific language governing permissions and
+- limitations under the License.
+-->
+
+# Auxiliary SQL Functions
+
+Kyuubi provides several auxiliary SQL functions as supplement to
+Flink's [Built-in Functions](https://nightlies.apache.org/flink/flink-docs-release-1.17/docs/dev/table/functions/systemfunctions/)
+
+|        Name         |                         Description                         | Return Type | Since |
+|---------------------|-------------------------------------------------------------|-------------|-------|
+| kyuubi_version      | Return the version of Kyuubi Server                         | string      | 1.8.0 |
+| kyuubi_engine_name  | Return the application name for the associated query engine | string      | 1.8.0 |
+| kyuubi_engine_id    | Return the application id for the associated query engine   | string      | 1.8.0 |
+| kyuubi_system_user  | Return the system user name for the associated query engine | string      | 1.8.0 |
+| kyuubi_session_user | Return the session username for the associated query engine | string      | 1.8.0 |
+

docs/extensions/engines/flink/index.rst

@@ -20,6 +20,7 @@ Extensions for Flink
     :maxdepth: 1
 
     ../../../connector/flink/index
+    functions
 
 .. warning::
    This page is still in-progress.