Add footer to page with pointer to this repo and preferred feedback mechanism

[tripu]

I've seen that users who detect issues don't know where to direct feedback.

Imitate the “standard” footer that we have been using in other projects (w3c/wbs-design#40).

If the footer is too far at the bottom of the page (the unfiltered version is many screenfuls long): put this copy in the sidebar, or at the top of the page?

[tripu]

At the same time, remove the notice at the top, which is old by now.