refactor: organize Oracle builtin functions docs into subdirectory

Author: yuanyl630

CN/modules/ROOT/nav.adoc

@@ -57,8 +57,8 @@
 **** xref:master/compatibility_features_design/empty_string_to_null.adoc[空字符串转null]
 **** xref:master/compatibility_features_design/call_into.adoc[CALL INTO]
 *** 内置函数
-**** xref:master/sys_context.adoc[sys_context]
-**** xref:master/userenv.adoc[userenv]
+**** xref:master/oracle_builtin_functions/sys_context.adoc[sys_context]
+**** xref:master/oracle_builtin_functions/userenv.adoc[userenv]
 *** xref:master/gb18030.adoc[国标GB18030]
 ** Oracle兼容功能列表
 *** xref:master/oracle_compatibility/compat_case_conversion.adoc[1、大小写转换]

…dules/ROOT/pages/master/sys_context.adoc …racle_builtin_functions/sys_context.adocCN/modules/ROOT/pages/master/sys_context.adoc renamed to CN/modules/ROOT/pages/master/oracle_builtin_functions/sys_context.adoc CN/modules/ROOT/pages/master/sys_context.adoc renamed to CN/modules/ROOT/pages/master/oracle_builtin_functions/sys_context.adoc

@@ -56,8 +56,8 @@
 *** xref:master/compatibility_features_design/empty_string_to_null.adoc[Empty String to NULL]
 *** xref:master/compatibility_features_design/call_into.adoc[CALL INTO]
 ** Built-in Functions
-*** xref:master/sys_context.adoc[sys_context]
-*** xref:master/userenv.adoc[userenv]
+*** xref:master/oracle_builtin_functions/sys_context.adoc[sys_context]
+*** xref:master/oracle_builtin_functions/userenv.adoc[userenv]
 ** xref:master/gb18030.adoc[GB18030 Character Set]
 * List of Oracle compatible features
 ** xref:master/oracle_compatibility/compat_case_conversion.adoc[1、Case conversion]