Add sample problem and GIF animation

[keurfonluu]

• Added: sample problem in gallery,
• Added: sample GIF animation.

Reminders:

• [x] Run invoke format to make sure the code follows the style guide,
• [x] Add tests for new features or tests that would have caught the bug that you're fixing,
• [x] Write detailed docstrings for all functions, classes and/or methods,
• [x] If adding new functionality, unit test it and add it to the documentation.

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@4b19054). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #8   +/-   ##
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  Coverage          ?   89.80%           
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  Files             ?       29           
  Lines             ?      942           
  Branches          ?        0           
=========================================
  Hits              ?      846           
  Misses            ?       96           
  Partials          ?        0           

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