Update/rebase our custom Knative override files with Docsy HEAD

[RichieEscarez]

We created several customizations to the original Docsy template (we started with an alpha/pre-released version): https://github.com/knative/website/tree/master/layouts

(Note: any file in the Knative layouts folder overrides the corresponding file from the Docsy template)

The Hugo and Docsy versions of knative.dev have been updated to recently. To override a template file, you copy the original template file and then add your customizations to those files.

Because many of the Docsy template files have changed and include enhancements, we need to migrate our customizations into recent versions of the corresponding Docsy template files.

(1)
Compare our Knative customization here: https://github.com/knative/website/tree/master/layouts
With the latest version of the Docsy template here: https://github.com/google/docsy/tree/master/layouts

(2)
"Rebase" our custom Knative template file for anything needed to pick up the latest Docsy features (banners, click-to-copy, etc.)