add rate limiting per engine

docs/admin/engines/settings.rst

@@ -473,6 +473,9 @@ engine is shown.  Most of the options have a default value or even are optional.
      max_connections: 100
      max_keepalive_connections: 10
      keepalive_expiry: 5.0
+     rate_limit:
+        - max_requests: 200
+          interval: 60
      proxies:
        http:
          - http://proxy1:8080
@@ -545,6 +548,15 @@ engine is shown.  Most of the options have a default value or even are optional.
   - ``ipv4`` set ``local_addresses`` to ``0.0.0.0`` (use only IPv4 local addresses)
   - ``ipv6`` set ``local_addresses`` to ``::`` (use only IPv6 local addresses)
 
+``rate_limit``: optional
+  Limit how many outgoing requests is SearXNG going to send to the engines.
+  It is a list where each element has:
+
+  - ``max_requests`` is the maximum number of requests that will be sent to this
+    engine per interval.
+  - ``interval`` (optional) is the number of seconds before this engine's rate
+    limiter is reset. Defaults to 1 second if unspecified.
+
 .. note::
 
    A few more options are possible, but they are pretty specific to some