Feature: Timestamping

### As a resident using the website, I want to see a timestamp on time-sensitive content so that I know how relevant the information is.

**Is your feature request related to a problem? Please describe.**
Some content is time-sensitive and updates often; for example, an alert on the page about measles in Austin or on a page explaining an ordinance that could change.

**Describe the solution you'd like**
The ability for content editors to opt in to a timestamp. Not sure if this would live on the page or content block level.

**Describe alternatives you've considered**
This is an early idea that surfaced during design pin-ups, so would be interesting to explore other options!

The issue names no files, tests, or entry points. Start by clarifying whether timestamps belong on pages or content blocks and how content editors opt in; done means selected time-sensitive content displays a timestamp for website residents.

Written by the indexing model from the issue text.