[Doc] Fix some MkDocs snippets used in the installation docs (#20572)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/getting_started/installation/cpu/apple.inc.md

@@ -54,9 +54,6 @@ If the build has error like the following snippet where standard C++ headers can
 ```
 
 # --8<-- [end:build-wheel-from-source]
-# --8<-- [start:set-up-using-docker]
-
-# --8<-- [end:set-up-using-docker]
 # --8<-- [start:pre-built-images]
 
 # --8<-- [end:pre-built-images]

docs/getting_started/installation/cpu/arm.inc.md

@@ -28,9 +28,6 @@ ARM CPU backend currently supports Float32, FP16 and BFloat16 datatypes.
 Testing has been conducted on AWS Graviton3 instances for compatibility.
 
 # --8<-- [end:build-wheel-from-source]
-# --8<-- [start:set-up-using-docker]
-
-# --8<-- [end:set-up-using-docker]
 # --8<-- [start:pre-built-images]
 
 # --8<-- [end:pre-built-images]

docs/getting_started/installation/cpu/s390x.inc.md

@@ -56,9 +56,6 @@ Execute the following commands to build and install vLLM from the source.
 ```
 
 # --8<-- [end:build-wheel-from-source]
-# --8<-- [start:set-up-using-docker]
-
-# --8<-- [end:set-up-using-docker]
 # --8<-- [start:pre-built-images]
 
 # --8<-- [end:pre-built-images]

docs/getting_started/installation/cpu/x86.inc.md

@@ -31,9 +31,6 @@ vLLM initially supports basic model inferencing and serving on x86 CPU platform,
     - If you want to force enable AVX512_BF16 for the cross-compilation, please set environment variable `VLLM_CPU_AVX512BF16=1` before the building.
 
 # --8<-- [end:build-wheel-from-source]
-# --8<-- [start:set-up-using-docker]
-
-# --8<-- [end:set-up-using-docker]
 # --8<-- [start:pre-built-images]
 
 See [https://gallery.ecr.aws/q9t5s3a7/vllm-cpu-release-repo](https://gallery.ecr.aws/q9t5s3a7/vllm-cpu-release-repo)