Add citation for Keçeci's 2025 publication

Added citation for Keçeci's work on Keçeci Numbers.

Author: WhiteSymmetry

docs/citation.rst

@@ -1 +1,16 @@
+Citation
+========
 
+.. code-block:: bibtex
+
+  misc{kececi_2025_15589625,
+    author       = {Keçeci, Mehmet},
+    title        = {Keçeci Numbers and the Keçeci Prime Number: A
+                     Potential Number Theoretic Exploratory Tool
+                    },
+    month        = May,
+    year         = 2025,
+    publisher    = {Zenodo},
+    doi          = {10.5281/zenodo.15381697},
+    url          = {https://doi.org/10.5281/zenodo.15381697},
+  }