docs(demo): add tail-based sampling example using service.criticality

[bachgarash]

This PR adds documentation for opentelemetry-demo which includes newly introduced semantic convention attribute service.criticality

Summary

• Add a new "Sample Configurations" section under the demo docs
• Document a tail-based sampling example using the service.criticality resource attribute introduced in SemConv 1.40.0
• Include collector configuration with sampling policies based on service criticality levels (critical, high, medium, low)

Context

This documentation accompanies open-telemetry/opentelemetry-demo#2950, which adds service.criticality as a resource attribute to all demo services. The example was originally included in the
demo's otelcol-config-extras.yml and moved here per reviewer feedback.

semconv PR

• I have read and followed the Contributing docs, especially the "First-time contributing?" section.
• This PR has content that I did not fully write myself.
  • I used AI and I have read and followed the Generative AI Contribution Policy.
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR.¹

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Footnotes

1. Yes, I can answer maintainer questions about the content of this PR, without using AI. ↩

[theletterf]

@open-telemetry/demo-maintainers Please have a look, thanks!