Improve documentation on how to replace Scene3D plugin

[j-rivero]

Following https://github.com/gazebosim/gz-gui/pull/485:

• Pointing to SDF code in Migration.md and reference it from the log message 781cecb87e90d3b94b3f362d7034d07e5fbec0e9
• Fix format in 781cecb87e90d3b94b3f362d7034d07e5fbec0e9

Fixes https://github.com/gazebosim/gz-gui/issues/484

[codecov[bot]]

Codecov Report

Merging #1698 (bb0f8b4) into gz-sim7 (ff1c82b) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           gz-sim7    #1698   +/-   ##
========================================
  Coverage    63.90%   63.90%           
========================================
  Files          334      334           
  Lines        26344    26344           
========================================
  Hits         16836    16836           
  Misses        9508     9508           

Impacted Files	Coverage Δ
src/gui/Gui.cc	68.22% <0.00%> (ø)

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