docs(core): update installation.md instructions for yarn (#28596)

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`yarn global` was removed in modern versions of yarn, this adds a hint
that it is only available in v1 of it.


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docs/shared/getting-started/installation.md

@@ -79,7 +79,7 @@ npm add --global nx@latest
 ```
 
 {% /tab %}
-{% tab label="yarn" %}
+{% tab label="yarn (v1 only)" %}
 
 ```shell
 yarn global add nx@latest