[DOCS] Moves reserved keywords under SQL section (#34124)

Author: lcawl

docs/reference/sql/appendix/syntax-reserved.asciidoc

@@ -1,8 +1,7 @@
 [role="xpack"]
 [testenv="basic"]
-[appendix]
 [[sql-syntax-reserved]]
-= Reserved keywords
+== Reserved keywords
 
 Table with reserved keywords that need to be quoted. Also provide an example to make it more obvious.