Add django banner to landing page

Author: jordan-smith721

snooty.toml

@@ -39,3 +39,13 @@ api-root = "https://pymongo.readthedocs.io/en/{+patch-version-number+}/api/"
 string-data-type = "``str``"
 int-data-type = "``int``"
 bool-data-type = "``bool``"
+django-odm = "Django MongoDB Backend"
+django-docs = "https://www.mongodb.com/docs/languages/python/django-mongodb/current"
+
+[[banners]]
+targets = ["index.txt"]
+variant = "tip"
+value = """
+    {+django-odm+} is now in Public Preview! \
+    To discover its features and learn more, see the `{+django-odm+} documentation <{+django-docs+}>`__.
+    """