docs(undergrad): add urls in linear algebra (#10415)
This uses the new possibility to put urls in `undergrad.yaml` hoping to help describing what is meant to be formalized. We should probably create wiki pages for some cases that are not so clear even with a url. There is one case where I could find only a French page and some cases where I couldn't find anything. Amazingly "endormorphism exponential" is such a case, but hopefully this example is already clear. Another kind of problematic item is the "example" item in the representation section. Presumably it should be removed and replaced with a couple of explicit examples such as "standard representation of a matrix group" or "permutation representation".
