[DOCS] Kubernetes Cluster receiver (k8s_cluster)

[alexandra5000]

What does this PR do?

This PR adds a new page documenting the Kubernetes Cluster receiver (k8s_cluster), a core component of the EDOT Collector.

The new page provides an overview of how the receiver works, configuration options, and common Elastic Observability use cases.

Why is it important?

The k8s_cluster receiver is included in the default EDOT Collector distribution but was not yet documented.

Checklist

• I have read and understood the pull request guidelines of this project.
  - [ ] My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works
  - [ ] I have added an entry in ./changelog/fragments using the changelog tool
  - [ ] I have added an integration test or an E2E test

Disruptive User Impact

No

How to test this PR locally

Follow the Contribute locally guide.

Related issues

• Closes #1027

[mergify]

This pull request does not have a backport label. Could you fix it @alexandra5000? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/edot-collector/components/k8sclusterreceiver.md