docs: update footer slot screenshots and simplify example plugin code

[brian-smith-tcril]

this updates the screenshots to show the footer in context with the home page, and removes the hide operation that isn't needed for replacement to work.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.89%. Comparing base (0435acd) to head (c33820a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #125   +/-   ##
=======================================
  Coverage   98.89%   98.89%           
=======================================
  Files         106      106           
  Lines        1083     1083           
  Branches      185      178    -7     
=======================================
  Hits         1071     1071           
  Misses         12       12           

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