Reorganize folders of DataScienceTutorials.jl

[EssamWisam]

This includes removing useless files, reorganizing common ones together, centralizing the routing index at the top-level and most importantly using a folder layout instead of prefixes to distinguish different tutorial types.

[EssamWisam]

Why does this heading exist? Old Instructions gives the impression that there are "alternative instructions"; meanwhile, in reality they do add information about exactly three things :

• How to visualize after changing (e.g., adding a new tutorial)
• Adding plots
• Troubleshooting

I would vote for the following:

• Get rid of the potentially misleading "Old Instructions" title
• So far, I added briefly how to visualize the changes as the last step in the "add tutorial section" instead I can link to here (which is less brief and more informative) or get rid of this section.
• The "Adding Plots" section is not absolutely necessary because it was mentioned earlier to follow the blueprint by copying another tutorial which makes that easy. Thus, we can either remove this section or keep it and link to it in the "adding tutorials section. In the latter case, we have to remove what's mentioned there on PyPlot
• Troubleshooting section should stay.

[ablaom]

@EssamWisam I'm very happy for you to make an executive decision about these details. Feel free to rewrite these instructions as much as you like.

Since the original developer has moved on, just go ahead do your best. You are the active maintainer with the most knowledge about the how the process now works and I will defer to you on this.

Do ping me when you are ready for final review.

[EssamWisam]

Alright. Ready for review. Here is a pretty print of the new README for easier review. The rest as can be seen are folder organization changes. I tested rendering the website after I did those.