Suppress warnings in validation notebook (#587) (#1696)

The original 2022 report ("Aggregates in full") referred to a section that
has since been rewritten away, and the current saved outputs are already
clean. Add an explicit warnings filter at the top of the notebook so a
future kernel re-run (e.g. pandas FutureWarning from pd.concat in a loop)
doesn't leak warning streams back into the docs.

Author: vahid-ahmadi

changelog.d/587.md

@@ -0,0 +1 @@
+- Suppress Python warnings at the top of the validation notebook so re-executions don't surface pandas/plotly deprecation noise in the docs build.

docs/book/validation/validation.ipynb

@@ -1789,6 +1789,9 @@
     }
    ],
    "source": [
+    "import warnings\n",
+    "warnings.filterwarnings(\"ignore\")\n",
+    "\n",
     "from policyengine_uk import Microsimulation\n",
     "\n",
     "sim = Microsimulation()\n",