Revise Datadog kubectl plugin documentation

[clamoriniere]

What does this PR do?

Update the kubectl plugin content to mention Helm chart installations

Motivation

Make it clear that the plugin can be use also with agent installed with the helm chart.

Additional Notes

Anything else we should know when reviewing?

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

Describe your test plan

Write there any instructions and details you may have to test your PR.

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.32%. Comparing base (490d736) to head (73050eb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2428   +/-   ##
=======================================
  Coverage   37.32%   37.32%           
=======================================
  Files         290      290           
  Lines       24720    24720           
=======================================
  Hits         9226     9226           
  Misses      14781    14781           
  Partials      713      713           

Flag	Coverage Δ
unittests	37.32% <ø> (ø)

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