docs: document helm custom value secret refs

[vigneshrajsb]

Summary

• document native Helm custom value secret references in the Cloud Secrets guide
• add examples for full-value secret refs, --set-file behavior, template ordering, and unsupported Codefresh Helm path
• use only dummy paths, service names, and example domains

Validation

• node_modules/.bin/prettier --check src/pages/docs/features/secrets.mdx
• bun run typecheck
• bun run lint