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>

layouts/docs/baseof.html

@@ -11,12 +11,13 @@
 			{{ partial "docs/top-menu.html" . }}
 		</section>
 		{{ end }}
+    {{ block "announcement" . }}{{ partial "announcement.html" . }}{{ end }}
 		{{ block "deprecation" . }}{{ partial "deprecation-warning.html" . }}{{ end }}
 		<section id="encyclopedia">
 			{{ block "side-menu" . }}<div id="docsToc" style="display:none;"></div>{{ end }}
 			<div id="{{ block "content-id" . }}docsContent{{ end }}">
         {{ block "content" . }}{{ end }}
-        
+
         {{ partial "feedback.html" . }}
 
         {{ partial "git-info.html" . }}