Ajouter Mermaid.js

Suite à #13 (comment 147314), je propose cette modification pour pouvoir intégrer les diagrammes MermaidJS dans les diapos.

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In Mermaid, we support the basic git operations like:

• commit : Representing a new commit on the current branch.
• branch : To create & switch to a new branch, setting it as the current branch.
• checkout : To checking out an existing branch and setting it as the current branch.
• merge : To merge an existing branch onto the current branch.

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Le code suivant :

gitGraph
   commit
   commit
   branch develop
   checkout develop
   commit
   commit
   checkout main
   merge develop
   commit
   commit

Donne :

gitGraph
   commit
   commit
   branch develop
   checkout develop
   commit
   commit
   checkout main
   merge develop
   commit
   commit