Add missing API documentation for mg_get_system_info

docs/api/mg_get_system_info.md

@@ -0,0 +1,25 @@
+# Civetweb API Reference
+
+### `mg_get_system_info( buffer, buflen );`
+
+### Parameters
+
+| Parameter | Type | Description |
+| :--- | :--- | :--- |
+|**`buffer**|`char *`|A string buffer to store the information|
+|**`buflen**|`int`|Size of the string buffer (including space for a terminating 0)|
+
+### Return Value
+
+| Type | Description |
+| :--- | :--- |
+|`int`|Available system information in bytes (excluding the terminating 0)|
+
+### Description
+
+The function `mg_get_system_info()` returns information collected for the system (operating system, compiler, version, ...). The string is in a human readable format - changes in the format are possible in future versions. This string should be included for support requests.
+
+### See Also
+
+* [`mg_get_context_info();`](mg_get_context_info.md)
+