Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

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

@@ -0,0 +1,13 @@
+---
+title: "mg_get_http_header()"
+decl_name: "mg_get_http_header"
+symbol_kind: "func"
+signature: |
+  struct mg_str *mg_get_http_header(struct http_message *hm, const char *name);
+---
+
+Searches and returns the header `name` in parsed HTTP message `hm`.
+If header is not found, NULL is returned. Example:
+
+    struct mg_str *host_hdr = mg_get_http_header(hm, "Host"); 
+