Add Kubernetes Resource Monitoring documentation

[amadeuszl]

Summary

It adds documentation about changes implemented due to dotnet/extensions#6496

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Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/built-in-metrics-diagnostics.md	.NET extensions metrics
docs/core/diagnostics/diagnostic-resource-monitoring.md	Diagnostic resource monitoring

[Copilot]

Pull request overview

This PR adds documentation and a runnable snippet that explains how to use Microsoft.Extensions.Diagnostics.ResourceMonitoring.Kubernetes to calculate resource monitoring metrics based on Kubernetes Downward API-provided limits/requests.

Changes:

• Added a new “Kubernetes resource monitoring” section to the existing resource monitoring article, including Downward API configuration guidance.
• Added a new runnable snippet project and minimal host-based sample that registers Kubernetes resource monitoring.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 12 comments.

File	Description
docs/core/diagnostics/diagnostic-resource-monitoring.md	Adds a Kubernetes-focused resource monitoring section, including configuration guidance, metric list, and example output.
docs/core/diagnostics/snippets/resource-monitoring-kubernetes/Program.cs	Adds a minimal host-based snippet showing AddKubernetesResourceMonitoring("MY_APP_").
docs/core/diagnostics/snippets/resource-monitoring-kubernetes/resource-monitoring-kubernetes.csproj	Adds a new snippet project referencing the Kubernetes resource monitoring package and hosting/logging dependencies.