add citation of the preprint

Author: mateuszbaran

docs/src/references.bib

@@ -87,6 +87,16 @@ @article{BacakBergmannSteidlWeinmann:2016
     VOLUME       = {38},
 }
 
+@misc{BaranBergmannPrzybysz:2026,
+	title = {A {Riemannian} quasi-{Newton} algorithm for optimization with {Euclidean} bounds},
+	doi = {10.48550/arXiv.2605.10573},
+	publisher = {arXiv},
+	author = {Baran, Mateusz and Bergmann, Ronny and Przybysz, Patryk},
+	month = may,
+	year = {2026},
+	note = {arXiv:2605.10573 [math.OC]},
+}
+
 @inproceedings{Beale:1972,
    ADDRESS = {London},
    AUTHOR = {Beale, E. M. L.},

docs/src/solvers/generalized_cauchy_direction_subsolver.md

@@ -1,6 +1,6 @@
 # Generalized Cauchy direction subsolver
 
-The generalized Cauchy direction (GCD) subsolver is a component in optimization algorithms that handle problems with bound constraints. It solves the following problem
+The generalized Cauchy direction (GCD) subsolver is a component in optimization algorithms that handle problems with bound constraints [BaranBergmannPrzybysz:2026](@cite). It solves the following problem
 
 ```math
 \begin{align*}