Add serving-internal docs about Knative encryption support

[ReToCode]

Related to #11906

Proposed Changes

• Add serving-internal docs about Knative encryption support
• Add a visualization

[ReToCode]

As discussed yesterday in Security WG.

/assign @evankanderson

[codecov]

Codecov Report

Base: 86.22% // Head: 86.25% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (c8c9ec8) compared to base (3b8d6cb).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13662      +/-   ##
==========================================
+ Coverage   86.22%   86.25%   +0.03%     
==========================================
  Files         197      197              
  Lines       14783    14783              
==========================================
+ Hits        12746    12751       +5     
+ Misses       1735     1731       -4     
+ Partials      302      301       -1     

Impacted Files	Coverage Δ
pkg/reconciler/configuration/configuration.go	84.36% <0.00%> (+1.42%)	⬆️
pkg/autoscaler/statserver/server.go	77.37% <0.00%> (+1.45%)	⬆️

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[evankanderson]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: evankanderson, ReToCode

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