Commonize mg_str

PUBLISHED_FROM=0e6a1c1a2356c1e12580e498c64ba7be367f601c

docs/c-api/net.h/intro.md

@@ -25,7 +25,6 @@ items:
   - { type: file, name: mg_set_timer.md }
   - { type: file, name: mg_time.md }
   - { type: file, name: mg_event_handler_t.md }
-  - { type: file, name: struct_mg_str.md }
   - { type: file, name: struct_mg_mgr.md }
   - { type: file, name: struct_mg_connection.md }
   - { type: file, name: struct_mg_add_sock_opts.md }

docs/c-api/net.h/struct_mg_str.md

@@ -1,13 +0,0 @@
----
-title: "struct mg_str"
-decl_name: "struct mg_str"
-symbol_kind: "struct"
-signature: |
-  struct mg_str {
-    const char *p; /* Memory chunk pointer */
-    size_t len;    /* Memory chunk length */
-  };
----
-
-Describes chunk of memory 
-