docs: clarify Kubernetes and runtime prerequisites before installing KubeEdge

[poox41]

What this PR does

This PR improves the installation documentation by clarifying that installing KubeEdge does not automatically install Kubernetes or the container runtime.

Why this is needed

New users may misunderstand keadm as a tool that provisions a full Kubernetes environment together with KubeEdge. In practice, Kubernetes and the runtime environment must be prepared separately before installing KubeEdge.

Changes made

• clarified the scope of keadm in the installation documentation
• added a reminder that Kubernetes and the runtime environment must be prepared before installation
• improved the getting started documentation to reduce this misunderstanding

Issue

Fixes #6679

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[gemini-code-assist]

Code Review

This pull request updates the documentation to clarify that KubeEdge requires a pre-existing Kubernetes and container runtime environment before installation. Feedback suggests consolidating redundant information in the keadm setup guide and adding a link to the runtime prerequisites in the getting started guide for better consistency and usability.

[gemini-code-assist]

The information regarding Kubernetes and runtime prerequisites is repeated twice in the first few lines. Consolidating this into a single, clear note improves readability and reduces redundancy.

Suggested change

	Keadm is used to install the cloud and edge components of KubeEdge. It does not install Kubernetes or the [runtime environment](https://kubeedge.io/docs/setup/prerequisites/runtime). Before using `keadm`, make sure your Kubernetes cluster and runtime environment are already prepared.
	## Important note
	Installing KubeEdge with `keadm` does not install Kubernetes or the container runtime. `keadm` only installs KubeEdge components. Before running `keadm init` or `keadm join`, make sure Kubernetes and the runtime environment are already installed and working properly.
	Keadm is used to install the cloud and edge components of KubeEdge.
	Note: keadm does not install Kubernetes or the [runtime environment](https://kubeedge.io/docs/setup/prerequisites/runtime). Before running keadm init or keadm join, make sure your Kubernetes cluster and container runtime are already installed and working properly.

[fujitatomoya]

i second this comment, this is pure duplication for user documentation.

[gemini-code-assist]

Adding a link to the runtime prerequisites page here would be consistent with the other installation documents and helpful for users who need to set up their environment.

Suggested change

	KubeEdge is installed on top of an existing Kubernetes environment. Installing KubeEdge does not automatically provision Kubernetes or the container runtime, so make sure they are prepared before installation.
	KubeEdge is installed on top of an existing Kubernetes environment. Installing KubeEdge does not automatically provision Kubernetes or the [container runtime](https://kubeedge.io/docs/setup/prerequisites/runtime), so make sure they are prepared before installation.

[fujitatomoya]

this PR includes the similar change with #784. can you merge the PR into one? actually, this PR does not really make sense to me though.

[fujitatomoya]

i second this comment, this is pure duplication for user documentation.