Add announcement banner html to enable config

docs/_static/announcement.html

@@ -0,0 +1,9 @@
+<!-- This will appear in the announcement banner at the top of the site.
+It needs to be activated by passing the link:
+"https://napari.org/dev/_static/announcement.html"
+in the html_theme_options section of conf.py using the key:
+"announcement"
+-->
+<div class="sidebar-message">
+    We want your feedback: take the <a href="https://bit.ly/napari-survey-2023-h">2023 annual survey</a> and help us improve napari for you and the community!
+</div>