Add Compiler Explorer to section educational

[Karneades]

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• Use a meaningful title for the pull request. Include the name of the resource added.
  Tool and section mentioned.
• Add a short sentence on why do you think the resource you're adding is awesome!
  It helps understanding and looking at the output of various compilers quickly and one is able to test different compilers without the hassle of installing them all. Furthermore, it's possible to test different compiler options and various versions of the same compiler easily. And it has a Vim-mode which is awesome.
• Make sure your addition maintains the alphabtical order of the list.
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I would split educational and toy projects or maybe I misunderstood the word "educational". It feels awkward adding Compiler Explorer to section toy project :) Would you move it to "Serious Projects" under Compilers and Interpreters or "Language Agnostic" under Tools and Frameworks?