Add mg_http_send_redirect and make examples use it

PUBLISHED_FROM=f45e06aeba1c00212078d539b6f74cb8bd054326

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

@@ -17,6 +17,7 @@ items:
   - { name: mg_sock_addr_to_str.md }
   - { name: mg_hexdump.md }
   - { name: mg_hexdump_connection.md }
+  - { name: mg_asprintf.md }
   - { name: mg_avprintf.md }
   - { name: mg_is_big_endian.md }
   - { name: mg_next_comma_list_entry.md }