docs: add citation for hash consing improvements

Include citation for "Efficient Symbolic Computation via Hash Consing"
(arXiv:2509.20534) documenting the hash consing optimizations
implemented in SymbolicUtils.jl [skip ci]

Author: bowenszhu

CITATION.bib

@@ -0,0 +1,13 @@
+Hash Consing Optimization: SymbolicUtils.jl now uses hash consing to eliminate duplicate 
+symbolic expressions in memory and enable faster equality comparisons. This optimization 
+significantly improves performance for complex symbolic computations by reducing memory 
+overhead and computation time. Available starting from v3.28.0.
+@misc{zhu2025efficientsymboliccomputationhash,
+    title={Efficient Symbolic Computation via Hash Consing}, 
+    author={Bowen Zhu and Aayush Sabharwal and Songchen Tan and Yingbo Ma and Alan Edelman and Christopher Rackauckas},
+    year={2025},
+    eprint={2509.20534},
+    archivePrefix={arXiv},
+    primaryClass={cs.PL},
+    url={https://arxiv.org/abs/2509.20534}, 
+}