Clarify and share Kubernetes OpenTelemetry setup

[brett0000FF]

What does this PR do? What is the motivation?

Makes the Kubernetes OpenTelemetry setup easier to find and follow while keeping one source for the instructions.

• Adds a Kubernetes navigation entry and a thin page that includes the existing OTel Kubernetes Metrics guide. Its edit link points to the source guide.
• Clarifies the setup's scope, prerequisites, Datadog site, namespace, configuration downloads, and verification steps.
• Keeps the infrastructure-metrics Collector configurations in opentelemetry-examples, using direct downloads instead of maintaining copies in Docs.
• Replaces the Explorer-only procedure's disconnected configuration blocks with one complete Helm values example. Automatic cluster-name detection and application correlation remain optional.
• Makes the choice between Explorer-only setup and infrastructure metrics clear before installation, and simplifies optional application tagging.

Merge readiness

• Ready for merge

For Datadog employees:

• ⚠️ Your branch name MUST follow the <name>/<description> convention and include the forward slash (/). If you've already created your PR with an incorrect branch name, please rename your branch and open a fresh PR.
• 🤖 New: Comment with /review to run an automated check that catches common issues before a Documentation team member reviews your PR.

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/brett.blue/otel-kubernetes-docs/containers/kubernetes/opentelemetry

Modified Files

• https://docs-staging.datadoghq.com/brett.blue/otel-kubernetes-docs/containers/kubernetes/
• https://docs-staging.datadoghq.com/brett.blue/otel-kubernetes-docs/containers/monitoring/kubernetes_explorer
• https://docs-staging.datadoghq.com/brett.blue/otel-kubernetes-docs/opentelemetry/integrations/kubernetes_metrics