Make footer sticky

Hi there!

I was about to suggest adding a footer partial, then I pulled the latest update to `main` and found you already added one. Perfect timing, and thanks!

I'd like to suggest making the footer 'sticky', specifically with the behaviour described and demonstrated [here](https://css-tricks.com/a-clever-sticky-footer-technique/), so it sticks to the bottom of the viewport *only* when there isn't enough page content to push it down, but *does* scroll down out of view rather than overlap page contents.

Start with the footer partial mentioned in the issue and review the linked CSS-Tricks sticky-footer technique. Verify the footer stays at the viewport bottom only when content is short, while longer pages let it scroll away without overlapping page contents.

Scritto dal modello di indicizzazione a partire dal testo della issue.