Make mg_http_serve_file public

A way to serve a single, specific file as opposed to the hairy monster
that is mg_serve_http.

PUBLISHED_FROM=6ed1bc487b9d41d212a5907337182f23b040aecf

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

@@ -26,6 +26,7 @@ items:
   - { type: file, name: mg_connect_http.md }
   - { type: file, name: mg_connect_http_opt.md }
   - { type: file, name: mg_serve_http.md }
+  - { type: file, name: mg_http_serve_file.md }
   - { type: file, name: mg_register_http_endpoint.md }
   - { type: file, name: mg_file_upload_handler.md }
   - { type: file, name: mg_http_check_digest_auth.md }

docs/c-api/http.h/mg_http_serve_file.md

@@ -0,0 +1,28 @@
+---
+title: "mg_http_serve_file()"
+decl_name: "mg_http_serve_file"
+symbol_kind: "func"
+signature: |
+  void mg_http_serve_file(struct mg_connection *nc, struct http_message *hm,
+                          const char *path, const struct mg_str mime_type,
+                          const struct mg_str extra_headers);
+---
+
+Serves a specific file with a given MIME type and optional extra headers.
+
+Example code snippet:
+
+```c
+static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
+  switch (ev) {
+    case MG_EV_HTTP_REQUEST: {
+      struct http_message *hm = (struct http_message *) ev_data;
+      mg_http_serve_file(nc, hm, "file.txt",
+                         mg_mk_str("text/plain"), mg_mk_str(""));
+      break;
+    }
+    ...
+  }
+}
+``` 
+