Standalone agent install with helm updated (elastic#1431)

This PR aligns the Helm Chart - Standalone Agent installation document
with the updates made recently for the Fleet Managed agent.

Pages updated:
- [Install Standalone Elastic Agent on Kubernetes using
Helm](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1431/reference/fleet/example-kubernetes-standalone-agent-helm)

We have updated several parts of the document.

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Co-authored-by: Fabrizio Ferri-Benedetti <[email protected]>

Author: eedugon