Add "Last updated" shortcode to Lotus Docs

[johnnymatthews]

Filecoin Docs has a small Last updated on April 5, 2023 link at the bottom of each page.

Following some UX research ran by the Filecoin Foundation, this link has been well received by testers. We should explore adding this feature to Lotus Docs. Since both sites are built from the same Hugo template, this should be super easy to port over.

[johnnymatthews]

I was wrong. The Lotus Docs site doesn't contain the logic from later docs-starter builds. I'll need to port the logic over from somewhere else, likely Filecoin Docs. Bumping up the difficulty of this to medium. Should still take less than a day to complete.