add documentation pointers to new doi

Again, it's great to have a DOI! In addition to the pointer somebody (presumably Adam) has already added in the project description (the thing on top in gitlab), I've also submitted MR !576 (merged) to point to it from the README.

As a third point, what would be the best top-level place to add "how to cite" information in the doxygen documentation? cc @karl-wette @adam-mercer (modulo #119 (closed))