[documentation] Secrets management in Operator 1.11+

[tbavelier]

What does this PR do?

Documents feature introduced by #1395

Motivation

What inspired you to submit this pull request?

Additional Notes

This requires Operator 1.11+

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 48.66%. Comparing base (c58579a) to head (55c06c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1555   +/-   ##
=======================================
  Coverage   48.66%   48.66%           
=======================================
  Files         226      226           
  Lines       20370    20370           
=======================================
  Hits         9913     9913           
  Misses       9940     9940           
  Partials      517      517           

Flag	Coverage Δ
unittests	48.66% <ø> (ø)

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[drichards-87]

Left some minor feedback from Docs and approved the PR.