Changed the array to a proper tikz-cd

]]>I have touched the formatting at *direct sum* and then expanded a little:

Added a paragraph to the Idea-section such that something familiar is mentioned right at the beginning;

Expanded on the example of direct sums in $Ab$ by drawing the cocone diagrams and explicitly mentioning the universal property.

Mentioned the relation to formal linear combinations.

Mentioned the examples of direct sums of modules.