Move cli args docs to its own page (#18228) (#18264)

Signed-off-by: Trevor Royer <troyer@redhat.com>

docs/source/serving/engine_args.md

@@ -7,6 +7,8 @@ Engine arguments control the behavior of the vLLM engine.
 - For [offline inference](#offline-inference), they are part of the arguments to `LLM` class.
 - For [online serving](#openai-compatible-server), they are part of the arguments to `vllm serve`.
 
+For references to all arguments available from `vllm serve` see the [serve args](#serve-args) documentation.
+
 Below, you can find an explanation of every engine argument:
 
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