Fix sample-config YAML quoting and refresh documentation

[saripurigopi]

What this PR does / why we need it

Fixes YAML parse errors in docs/sample-config.yaml caused by double-quoted
metricsQuery values containing inner double quotes (fixes #697).

Updates configuration documentation for accuracy and clarity (fixes #679):

• Add YAML quoting guidance and link to working sample config
• Fix incorrect metric names in config-walkthrough discovery example
• Update walkthrough Kubernetes version references

Which issue(s) this PR fixes

Fixes #697
Fixes #679

Special notes for your reviewer

Documentation-only change. Verified docs/sample-config.yaml parses with PyYAML.

NONE

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: saripurigopi
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