Changed documentation backup folder and added generated documentation to docs/

This change is a preparation to migrate the documentation from the gh-pages branch to the docs/ folder in the master branch (once dev is merged)

docs/includes/multiset

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+<h1 id='multiset-h'>multiset.h</h1>
+<p>In mathematics, a multiset is a modification of the concept of a set that, unlike a set, allows for multiple instances for each of its elements. The positive integer number of instances, given for each element is called the multiplicity of this element in the multiset. A MultiSet also has a cardinality which equals the sum of the multiplicities of its elements.</p>