Revise the Kubernetes website to use Docsy (#20874)

* Add Docsy, required packages and config

* Apply Docsy integration work to k8s website

This encompasses significant changes to the style system, layout files,
as well as changes to some of the static assets. To examine this work
and the iterations that were performed, the original repository can be
found at https://github.com/gearbox-built/kubernetes-hugo.git

* Address issues found in review

The styling and positioning of the header, footer, some shortcodes and
main container elements are improved. A new partial is added for
favicons. The sidebars, as well as inner table of contents, have been
altered for better positioning and rendering of content. Localization
has also been addressed.

* Enable announcements

The recent Black Lives Matter announcement surfaced issues which have
been addressed.

Author: aidanranney

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "themes/docsy"]
+	path = themes/docsy
+	url = https://github.com/google/docsy.git