docs: add a note explaining the need for topolvm.io/webhook=ignore

In getting started, it is not clear why `topolvm.io/webhook=ignore` is
needed. Add a note explaining it.

Closes: #907

Signed-off-by: Yuma Ogami <yuma-ogami@cybozu.co.jp>

docs/getting-started.md

@@ -24,6 +24,9 @@ kubectl label namespace topolvm-system topolvm.io/webhook=ignore
 kubectl label namespace kube-system topolvm.io/webhook=ignore
 ```
 
+> [!NOTE]
+> The label `topolvm.io/webhook=ignore` is set here to prevent the triggering of TopoLVM's mutating webhooks when pods and pvcs are created in the topolvm-system and kube-system namespaces. These webhooks depend on the topolvm-controller. Therefore, the webhooks should not be activated during TopoLVM's startup process to avoid causing it to become stuck.
+
 Then, install TopoLVM with the release name `topolvm`.
 
 ```sh