fix(docs): fix broken Summary API link and add kubelet metrics endpoints

[prathamesh04]

What this PR does

Fixes #56087 - Fixes broken Summary API link and adds kubelet metrics endpoints documentation.

Changes made

1. Fix broken Summary API link

• Updated link from /docs/reference/config-api/kubelet-stats.v1alpha1/ (404) to https://pkg.go.dev/k8s.io/kubelet/pkg/apis/stats/v1alpha1
• Fixed in both node-metrics.md and understand-psi-metrics.md

2. Add kubelet metrics endpoints documentation

Added a new section documenting kubelet metrics endpoints:

Endpoint	Description	Format
/stats/summary	Summary API with node, pod, and container stats	JSON
/metrics/resource	Resource metrics (CPU, memory) for pods and containers	Prometheus
/metrics/cadvisor	Detailed container metrics from cAdvisor	Prometheus
/metrics	Kubelet's own operational metrics	Prometheus

Why this change

The issue reported a broken link on the node-metrics page where the Summary API link returned a 404. Additionally, the page lacked clear documentation about available kubelet metrics endpoints, making it difficult for users to understand how to access node metrics.

Files changed

• content/en/docs/reference/instrumentation/node-metrics.md - Fixed broken link, added metrics endpoints section
• content/en/docs/reference/instrumentation/understand-psi-metrics.md - Fixed broken link

Verification

• All Summary API links now point to valid Go package documentation
• New metrics endpoints section follows Kubernetes documentation style guide
• No remaining broken links to kubelet-stats.v1alpha1

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[lmktfy]

I am not sure we should link to the Go API reference.

Maybe there's no better option but (I believe that) actually the wire format is what we expect for conformance etc, not the Golang API.

[lmktfy]

@prathamesh04 did you generate this PR using AI?

If you did, we (Kubernetes project) ask and expect you to disclose this in the PR description.