Add deprecation notice to the old variational equations tutorial.

doc/notebooks/The variational equations.ipynb

@@ -5,12 +5,17 @@
    "id": "excited-uganda",
    "metadata": {},
    "source": [
+    "```{note}\n",
+    "\n",
+    "Starting with version 5.0.0, heyoka.py features builtin support for the formulation of the [variational equations](./var_ode_sys.ipynb). This tutorial is now deprecated and it will be removed in a future version.\n",
+    "\n",
+    "```\n",
+    "\n",
     "The variational equations\n",
     "=========================\n",
     "\n",
     "In this tutorial, we will show how it is possible to exploit heyoka.py's [expression system](<./The expression system.ipynb>) to implement first-order variational equations for a system of ODEs.\n",
     "\n",
-    "\n",
     "First, let's start with a brief recap on the variational equations. For simplicity, we will consider an autonomous ODE system in the variables $x$ and $y$ (the extension to more variables just complicates the notation):\n",
     "\n",
     "$$\n",