Add LU solver documentation

docs/advanced_documentation/algorithms/index.md

@@ -0,0 +1,12 @@
+<!--
+SPDX-FileCopyrightText: Contributors to the Power Grid Model project <powergridmodel@lfenergy.org>
+
+SPDX-License-Identifier: MPL-2.0
+-->
+
+# Algorithms
+
+```{toctree}
+self
+lu-solver
+```