add reference to Yusha's paper in ebnm_generalized_binary docs

[willwerscheid]

do this once it is publicly available somewhere

[willwerscheid]

also update the reference to the grid selection paper when that's done

[willwerscheid]

should add new version of ebnm paper as vignette too, if possible

[willwerscheid]

reference to yusha's paper has been added; i am keeping this open to remember to do the other references