[Doc][4/N] Reorganize API Reference (#11843)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/source/api/engine/async_llm_engine.md

@@ -0,0 +1,7 @@
+# AsyncLLMEngine
+
+```{eval-rst}
+.. autoclass:: vllm.AsyncLLMEngine
+    :members:
+    :show-inheritance:
+```

docs/source/api/engine/index.md

@@ -0,0 +1,17 @@
+# vLLM Engine
+
+```{eval-rst}
+.. automodule:: vllm.engine
+```
+
+```{eval-rst}
+.. currentmodule:: vllm.engine
+```
+
+```{toctree}
+:caption: Engines
+:maxdepth: 2
+
+llm_engine
+async_llm_engine
+```

docs/source/api/engine/llm_engine.md

@@ -0,0 +1,7 @@
+# LLMEngine
+
+```{eval-rst}
+.. autoclass:: vllm.LLMEngine
+    :members:
+    :show-inheritance:
+```