DOC: replace Beginners Guide pluto notebook with simple markdown html.

[juanjospina]

Pull Request (PR) Template

This PR replaces the currently used BeginnersGuide.jl Pluto notebook with a simple HTML file (with markdown) that presents the Beginner guide in a more concise and simple way.

This PR is needed due to the error that exists when running PowerModelsITD using Pluto notebooks. There is a compatibility error inherited from PowerModels, and that is related to the Memento logger, that causes a stream error in Pluto Notebooks, making it impossible to run PowerModelsITD code in a Pluto notebook. So, to avoid confusing the user (beginner), we removed the need to use a Pluto notebook to follow the Beginner's guide.

This PR removes Gumbo and Pluto dependencies previously needed to create the DOCs of the project.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (76ad53e) 80.57% compared to head (93877e9) 80.57%.

Additional details and impacted files

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##             main      #20   +/-   ##
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  Coverage   80.57%   80.57%           
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  Files          30       30           
  Lines        4885     4885           
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  Hits         3936     3936           
  Misses        949      949           

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