chore(toolkit-lib): add assets directory to documentation build

[mrgrain]

Updates the projen configuration to include the assets directory in the documentation build process.

The changes:

1. Adds a step to copy files from the assets directory to the documentation output
2. Updates the docs task in .projen/tasks.json to include this new step

This ensures that any assets (like images or other resources) used in the documentation are properly included in the final documentation package.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.00%. Comparing base (a1ef4fc) to head (d1fd6ee).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #560   +/-   ##
=======================================
  Coverage   79.00%   79.00%           
=======================================
  Files          47       47           
  Lines        7083     7083           
  Branches      790      791    +1     
=======================================
  Hits         5596     5596           
  Misses       1468     1468           
  Partials       19       19           

Flag	Coverage Δ
suite.unit	79.00% <ø> (ø)

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