Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/c-api/http_server.h/mg_fu_fname_fn.md

@@ -0,0 +1,11 @@
+---
+title: "mg_fu_fname_fn"
+decl_name: "mg_fu_fname_fn"
+symbol_kind: "typedef"
+signature: |
+  typedef struct mg_str (*mg_fu_fname_fn)(struct mg_connection *nc,
+                                          struct mg_str fname);
+---
+
+Callback prototype for `mg_file_upload_handler()`. 
+