docs: change udf and faq level, add sql guide (#3534)

* docs: change udf and faq level, add sql guide

* Update beginner_must_read.md

---------

Co-authored-by: LU MIAN <[email protected]>

Author: vagetablechicken

.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 renamed to 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

docs/en/reference/sql/functions_and_operators/operators.md renamed to docs/en/reference/sql/operators/operators.md

@@ -8,6 +8,5 @@
     install_deploy
     conf
     compile
-    integrate_hadoop
     offline_integrate_kubernetes
     [Alpha]在线引擎基于 Kubernetes 部署 <https://github.com/4paradigm/openmldb-k8s>

docs/en/reference/sql/functions_and_operators/Files/udfs_8h.md renamed to docs/en/reference/sql/udfs_8h.md

@@ -1034,10 +1034,9 @@ RegisterUdafTemplate<DistinctCountDef>("distinct_count")
 
 ## 6. 文档管理
 
-内置函数文档可在 [Built-in Functions](https://openmldb.ai/docs/zh/main/openmldb_sql/functions_and_operators/Files/udfs_8h.html) 查看，它是一个代码生成的 markdown 文件，注意请不要进行直接编辑。
+内置函数文档可在 [Built-in Functions](../openmldb_sql/udfs_8h.md) 查看，它是一个代码生成的 markdown 文件，注意请不要进行直接编辑。
 
-- 如果需要对新增加的函数添加文档，请参照 2.2.4 配置函数文档 章节，说明了内置函数的文档是在 CPP 源代码中管理的。后续会通过一系列步骤生成如上网页中更加可读的文档， 即`docs/*/openmldb_sql/functions_and_operators/`目录下的内容。
+- 如果需要对新增加的函数添加文档，请参照 2.2.4 配置函数文档 章节，说明了内置函数的文档是在 CPP 源代码中管理的。后续会通过一系列步骤生成如上网页中更加可读的文档， 即`docs/*/openmldb_sql/`目录下的内容。
 - 如果需要修改一个已存在函数的文档，可以在文件 `hybridse/src/udf/default_udf_library.cc` 或者 `hybridse/src/udf/default_defs/*_def.cc` 下查找到对应函数的文档说明，进行修改。
 
 OpenMLDB 项目中创建了一个定期天级别的 GitHub Workflow 任务来定期更新这里的相关文档。因此内置函数文档相关的改动只需按照上面的步骤修改对应源代码位置的内容即可，`docs` 目录和网站的内容会随之定期更新。具体的文档生成流程可以查看源代码路径下的 [udf_doxygen](https://github.com/4paradigm/OpenMLDB/tree/main/hybridse/tools/documentation/udf_doxygen)。
-