Add a last edited date to source documents

[willingc]

We know that some docs are out of date. It would be helpful to display an accurate "last edited date" on a page.

@pradyunsg Any ideas on how to do that efficiently?

[pradyunsg]

https://github.com/mgeier/sphinx-last-updated-by-git

https://github.com/jdillard/sphinx-gitstamp

Both of these extensions gather git information from the repo and inject that into the build. I still have a TODO over at Lutra to properly deep dive into this aspect and present it.

IIRC, the first one should be easier to place into the docs without theme changes, and the latter would be a better fit for something to integrate into a theme/via templates.