Add announcement shortcode and partial (#16210)

Add shortcode, partial, and css so we can display an accouncement
site-wide.
Create a shortcode for use in content/en/ _index.html so
announcement is displayed on website landing page.
Create a partial for use in layouts/docs/baseof.html so
announcement is displaye on every page in '<url>/docs/home'
directory.
Add two values to config.toml: announcement (boolean) for toggling
display of announcement; announcement_message, which is the actual
message.

Signed-off-by: Aimee Ukasick <aimeeu.opensource@gmail.com>

content/en/_index.html

@@ -3,6 +3,7 @@ title: "Production-Grade Container Orchestration"
 abstract: "Automated container deployment, scaling, and management"
 cid: home
 ---
+{{< announcement >}}
 
 {{< deprecationwarning >}}