[8.16] Add docs for installing agent using Helm (backport #1375)

[mergify]

This adds docs for installing agent on Kubernetes using Helm.

Notes:

• For 8.16 these install instructions point to the Helm chart in elastic-agent/deploy/helm/elastic-agent. Once the chart has been published, please let me know and I'll update the commands accordingly.
• I've assumed that for 8.16 the default for --set agent.unprivileged will be true.
• The docs are all marked as technical preview.
• There's no hurry on reviewing; I can publish the docs anytime up to the 8.16 GA.
• Huge thanks to Panos for the excellent demo from which all of these steps were stolenborrowed. :-)

Preview pages:

• Install Elastic Agent on Kubernetes using Helm (overview with links to the following walk-throughs):
  • Example: Install standalone Elastic Agent on Kubernetes using Helm
  • Example: Install Fleet-managed Elastic Agent on Kubernetes using Helm

Closes: #1304

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This is an automatic backport of pull request #1375 done by [Mergify](https://mergify.com).

[github-actions]

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