Add Last Modified datestamp to page template

[ryanhofdotgov]

Briefly discussed in 01/31/2020 guild meeting:
As a reader of the guide, I want to easily determine how recently a particular page has been updated, as an indicator of relevancy.

[ryanhofdotgov]

Existing uswds-jekyll mechanism does not work on Federalist due to shallow checkout done there. Relevant Slack thread.

[ryanhofdotgov]

Federalist just got new feature that will allow jekyll-last-modified-at to work!
https://federalist.18f.gov/documentation/federalist-json/

[Sgtpluck]

Closing because this handbook is being merged into the TTS handbook, rendering it moot.

[mahmod87]

.