Links between the user guide and the reference would be nice

Refer to the original discussion: https://github.com/sebcrozet/nalgebra/issues/263

nalgebra provides two sources of documentation. The user guide is meant as a high-level description of the library, and is a lots more approachable when trying to discover its functionality, whereas the API documentation provides a more detailed reference suitable for answering more specific questions such as "why won't this code compile".

In this sense, these two forms of documentation are complementary. But currently, they don't link to one another. If I want to know more about a function described in the user guide, I need to manually look it up in the API documentation, through lots of copy and paste in the search box.

It would be nice if there were a way to automatically add links to the API reference in the user guide, so that one could just click on the name of the function in the user guide instead.