feat(banner): add top banner (#777)

* feat: add top banner

* chore: use inside the example site

* chore: generate

* fix: banner with the burger navbar

* fix: compute the banner height to allow mutliple lines

* chore: better p style

exampleSite/content/docs/guide/configuration.md

@@ -423,3 +423,22 @@ params:
     - "/img/config-image.jpg"
   audio: "config-talk.mp3"
 ```
+
+### Banner
+
+To add a banner to your site, add the following to your `hugo.yaml`:
+
+```yaml
+params:
+  banner:
+    key: 'announcement-xxx'
+    message: |
+      🎉 Welcome! [Hextra](https://github.com/hextra/hextra) is a static site generator that helps you build modern websites.
+```
+
+The banner will be displayed on all pages.
+
+The field `message` supports Markdown syntax.
+
+If you want to use template syntax, you can define the partial in `layouts/_partials/custom/banner.html`.
+In this case, the field `message` will be ignored.