Add a sphinx extension to show last edit date for each published page

[stichbury]

Description

I think there's a sphinx extension to add a note to each page so it's clear to the reader when the page was last updated and whether that was a major/minor change. I'm sure I've used one in the past, and found this Github repo, but it doesn't look like what I had enabled previously. Maybe that was when I was working on Jekyll instead.

It may be useful to include this while we're making some page improvements (and while we are still using Sphinx), so our readers can (a) see where pages are newer and (b) see where they're looking at legacy pages with a few minor tweaks or complete page rewrites/new pages.

There are some caveats to this such as when we change the name of each and every page to remove underscores, at which point it becomes less than useful.

Wanted to discuss whether it is worth adding this, but I don't see it as a high priority, just a useful "nice to have" add-on.

[stichbury]

Done using an extension in #3547