Add documentation for enable-dynamic-certificates feature

[hnrytrn]

What this PR does / why we need it:

Adds documentation for dynamic certificate serving feature.

[codecov-io]

Codecov Report

Merging #2983 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2983   +/-   ##
=======================================
  Coverage   47.54%   47.54%           
=======================================
  Files          77       77           
  Lines        5639     5639           
=======================================
  Hits         2681     2681           
  Misses       2605     2605           
  Partials      353      353

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[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, hnrytrn

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[aledbf]

@hnrytrn thanks for the this feature and the docs