add system page

Author: perazz

doc/specs/index.md

@@ -37,6 +37,7 @@ This is an index/directory of the specifications (specs) for each new module/fea
  - [string\_type](./stdlib_string_type.html) - Basic string support
  - [stringlist_type](./stdlib_stringlist_type.html) - 1-Dimensional list of strings
  - [strings](./stdlib_strings.html) - String handling and manipulation routines
+ - [system](./stdlib_system.html) - OS and sub-processing routines
  - [version](./stdlib_version.html) - Version information
 
 ## Released/Stable Features & Modules

doc/specs/stdlib_system.md

@@ -2,7 +2,10 @@
 title: system
 ---
 
-# System
+# System and sub-processing module
+
+The `stdlib_system` module provides interface for interacting with external processes, enabling the execution 
+and monitoring of system commands or applications directly from Fortran. 
 
 [TOC]