[SPARK-31434][SQL][DOCS] Drop builtin function pages from SQL references

### What changes were proposed in this pull request?

This PR intends to drop the built-in function pages from SQL references. We've already had a complete list of built-in functions in the API documents.

See related discussions for more details:
#28170 (comment)

### Why are the changes needed?

For better SQL documents.

### Does this PR introduce any user-facing change?

![functions](https://user-images.githubusercontent.com/692303/79109009-793e5400-7db2-11ea-8cb7-4c3cf31ccb77.png)

### How was this patch tested?

Manually checked.

Closes #28203 from maropu/DropBuiltinFunctionDocs.

Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit 853c6c9)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: maropu

docs/_data/menu-sql.yaml

@@ -242,23 +242,11 @@
     - text: Functions
       url: sql-ref-functions.html
       subitems:
-      - text: Build-in Functions
-        url: sql-ref-functions-builtin.html
-        subitems:
-        - text: Build-in Aggregate Functions
-          url: sql-ref-functions-builtin-aggregate.html
-        - text: Build-in Array Functions
-          url: sql-ref-functions-builtin-array.html
-        - text: Build-in Date Time Functions
-          url: sql-ref-functions-builtin-date-time.html
-      - text: UDFs (User-Defined Functions)
-        url: sql-ref-functions-udf.html
-        subitems:
-        - text: Scalar UDFs (User-Defined Functions)
-          url: sql-ref-functions-udf-scalar.html
-        - text: UDAFs (User-Defined Aggregate Functions)
-          url: sql-ref-functions-udf-aggregate.html
-        - text: Integration with Hive UDFs/UDAFs/UDTFs
-          url: sql-ref-functions-udf-hive.html
+      - text: Scalar UDFs (User-Defined Functions)
+        url: sql-ref-functions-udf-scalar.html
+      - text: UDAFs (User-Defined Aggregate Functions)
+        url: sql-ref-functions-udf-aggregate.html
+      - text: Integration with Hive UDFs/UDAFs/UDTFs
+        url: sql-ref-functions-udf-hive.html
     - text: Datetime Pattern
       url: sql-ref-datetime-pattern.html

docs/sql-ref-functions-builtin-aggregate.md

@@ -19,4 +19,13 @@ license: |
   limitations under the License.
 ---
 
-Spark SQL defines commonly used functions as built-in functions. It also provides User-Defined Functions (UDFs) to allow users to define their own functions when the system’s built-in functions are not enough to perform the desired task.
+Spark SQL provides two function features to meet a wide range of user needs: built-in functions and user-defined functions (UDFs).
+Built-in functions are commonly used routines that Spark SQL predefines and a complete list of the functions can be found in the [Built-in Functions](api/sql/) API document. UDFs allow users to define their own functions when the system’s built-in functions are not enough to perform the desired task.
+
+### UDFs (User-Defined Functions)
+
+User-Defined Functions (UDFs) are a feature of Spark SQL that allows users to define their own functions when the system's built-in functions are not enough to perform the desired task. To use UDFs in Spark SQL, users must first define the function, then register the function with Spark, and finally call the registered function. The User-Defined Functions can act on a single row or act on multiple rows at once. Spark SQL also supports integration of existing Hive implementations of UDFs, UDAFs and UDTFs.
+
+ * [Scalar User-Defined Functions (UDFs)](sql-ref-functions-udf-scalar.html)
+ * [User-Defined Aggregate Functions (UDAFs)](sql-ref-functions-udf-aggregate.html)
+ * [Integration with Hive UDFs/UDAFs/UDTFs](sql-ref-functions-udf-hive.html)