adding comprehensive readme documentation for plugin slots

[jacobo-dominguez-wgu]

Description

Created docs for pending plugin slots with example code and image.

Content IFrame Loader Slot
Course Outline Tab Notifications Slot
Gated Unit Content Message Slot
Notification Tray Slot
Notification Widget Slot

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.38%. Comparing base (06902d8) to head (ac1c9fe).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1770      +/-   ##
==========================================
+ Coverage   90.33%   90.38%   +0.04%     
==========================================
  Files         344      344              
  Lines        5793     5811      +18     
  Branches     1394     1396       +2     
==========================================
+ Hits         5233     5252      +19     
+ Misses        541      540       -1     
  Partials       19       19              

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[brian-smith-tcril]

Thanks for adding all this documentation!