docs(aws): document EKS resource detector prerequisites

[popsiclelmlm]

Description

Fixes #1384.

Summary

Adds AWS EKS resource detector prerequisite documentation to the AWS SDK extension README.

Reproduction

Users currently need to inspect AwsEksResourceDetector source to learn that it uses the mounted Kubernetes service account token and reads the cluster-info ConfigMap in the amazon-cloudwatch namespace.

Root cause

The README described AWS resource detectors generically but did not document the EKS detector's Kubernetes token, CA, RBAC, or ConfigMap requirements.

Changes

• Documents the mounted service account token/CA requirement.
• Adds minimal RBAC for reading the amazon-cloudwatch/cluster-info ConfigMap.
• Shows the required cluster-info ConfigMap field used by the detector.

Tests

• git diff --check
• /tmp/otel-rstcheck-venv/bin/rstcheck sdk-extension/opentelemetry-sdk-extension-aws/README.rst

Screenshots/Logs

Not applicable; docs-only change.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated (not applicable; docs-only README change)
• Unit tests have been added (not applicable; docs-only README change)
• Documentation has been updated

[herin049]

Thank you for this!