Add security announcement to docs site

docs/getting-started/installing.md

@@ -64,6 +64,17 @@ Once you've activated the virtual environment, you will see ".venv" at the start
 pip install mesop
 ```
 
+## Upgrading
+
+To upgrade Mesop, run the following command:
+
+```sh
+pip install --upgrade mesop
+```
+
+If you are using `requirements.txt` or `pyproject.toml` to manage your dependency versions, then you should update those.
+
+
 ## Next steps
 
 Follow the quickstart guide to learn how to create and run a Mesop app:

docs/theme/main.html

@@ -0,0 +1,7 @@
+{% extends "base.html" %} {% block announce %} Please
+<a href="/mesop/getting-started/installing/">upgrade</a> to the latest version
+of Mesop which addresses an important
+<a
+  href="https://github.com/google/mesop/security/advisories/GHSA-pmv9-3xqp-8w42"
+  >security issue.</a
+>{% endblock %}