Add not about sprockets manifest before running rails commands

[tvdeyen]

Summary

Due to a bug in sprockets-rails[1] we need to manually add the sprockets manifest into the generated rails app before running any rails commands inside the rails app folder.

1

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.81%. Comparing base (4a892c3) to head (eaf4f5f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6130   +/-   ##
=======================================
  Coverage   88.81%   88.81%           
=======================================
  Files         835      835           
  Lines       18146    18146           
=======================================
  Hits        16116    16116           
  Misses       2030     2030           

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