docs: change udf and faq level, add sql guide

.github/workflows/udf-doc.yml

@@ -54,8 +54,8 @@ jobs:
         if: github.event_name != 'pull_request'
         with:
           add-paths: |
-            docs/en/reference/sql/functions_and_operators/Files/udfs_8h.md
-            docs/zh/openmldb_sql/functions_and_operators/Files/udfs_8h.md
+            docs/en/reference/sql/udfs_8h.md
+            docs/zh/openmldb_sql/udfs_8h.md
           labels: |
             udf
           branch: docs-udf-patch

docs/en/developer/built_in_function_develop_guide.md

@@ -792,7 +792,7 @@ select date(timestamp(1590115420000)) as dt;
 
 ## 5. Document Management
 
-Documents for all built-in functions can be found in [Built-in Functions](http://4paradigm.github.io/OpenMLDB/zh/main/reference/sql/functions_and_operators/Files/udfs_8h.html). It is a markdown file automatically generated from source, so please do not edit it directly.
+Documents for all built-in functions can be found in [Built-in Functions](http://4paradigm.github.io/OpenMLDB/zh/main/reference/sql/udfs_8h.html). It is a markdown file automatically generated from source, so please do not edit it directly.
 
 - If you are adding a document for a new function, please refer to [2.2.4 Documenting Function](#224-documenting-function). 
 - If you are trying to revise a document of an existing function, you can find source code in the files of `hybridse/src/udf/default_udf_library.cc` or `hybridse/src/udf/default_defs/*_def.cc` .

docs/en/developer/udf_develop_guide.md

@@ -9,7 +9,7 @@ SQL functions can be categorised into scalar functions and aggregate functions.
 #### 2.1.1 Naming Specification of C++ Built-in Function
 - The naming of C++ built-in function should follow the [snake_case](https://en.wikipedia.org/wiki/Snake_case) style.
 - The name should clearly express the function's purpose.
-- The name of a function should not be the same as the name of a built-in function or other custom functions. The list of all built-in functions can be seen [here](../reference/sql/functions_and_operators/Files/udfs_8h.md).
+- The name of a function should not be the same as the name of a built-in function or other custom functions. The list of all built-in functions can be seen [here](../reference/sql/udfs_8h.md).
 
 #### 2.1.2 
 The types of the built-in C++ functions' parameters should be BOOL, NUMBER, TIMESTAMP, DATE, or STRING.

docs/en/reference/sql/dql/WINDOW_CLAUSE.md

@@ -320,5 +320,5 @@ WINDOW w1 AS (PARTITION BY col1 ORDER BY col5 ROWS_RANGE BETWEEN 10s PRECEDING A
 ```
 
 ```{seealso}
-Please refer to [Built-in Functions](../functions_and_operators/Files/udfs_8h.md) for aggregate functions that can be used in window computation.
+Please refer to [Built-in Functions](../udfs_8h.md) for aggregate functions that can be used in window computation.
 ````

docs/en/reference/sql/index.rst

@@ -9,6 +9,7 @@ SQL
     language_structure/index 
     data_types/index
     functions_and_operators/index
+    udfs_8h
     dql/index
     dml/index
     ddl/index

docs/en/reference/sql/functions_and_operators/index.rst → docs/en/reference/sql/operators/index.rst

@@ -1,10 +1,9 @@
 =============================
-Expressions, Functions, and Operations
+Expressions and Operations
 =============================
 
 
 .. toctree::
     :maxdepth: 1
 
     operators
-    Files/udfs_8h