[Misc][Docs] Raise error when flashinfer is not installed and VLLM_ATTENTION_BACKEND is set (#12513)

Signed-off-by: NickLucche <nlucches@redhat.com>

docs/source/getting_started/quickstart.md

@@ -184,3 +184,13 @@ chat_response = client.chat.completions.create(
 )
 print("Chat response:", chat_response)
 ```
+
+## On Attention Backends
+
+Currently, vLLM supports multiple backends for efficient Attention computation across different platforms and accelerator architectures. It automatically selects the most performant backend compatible with your system and model specifications.
+
+If desired, you can also manually set the backend of your choice by configuring the environment variable `VLLM_ATTENTION_BACKEND` to one of the following options: `FLASH_ATTN`, `FLASHINFER` or `XFORMERS`.
+
+```{attention}
+There are no pre-built vllm wheels containing Flash Infer, so you must install it in your environment first. Refer to the [Flash Infer official docs](https://docs.flashinfer.ai/) or see [Dockerfile](https://github.com/vllm-project/vllm/blob/main/Dockerfile) for instructions on how to install it.
+```