Add guidelines for showing timestamps ⏱

[maximedegreve]

It's time ⏱ for another issue. I could't find any information regarding usage of timestamps and thought this could make a great addition to our docs in the future.

🔦 Some examples I've found:

• Activity: 12d
• Issues: 16 Apr 2021
• PR deleted branch: 7 months ago
• Profile repositories: Updated on 27 Sep 2021
• Explore: Updated on 26 Feb
• Codespaces: Last used about 1 month ago
• Notifications: on 5 Jan

🎉 What I expect in the docs:

• Are there components/libraries available similar to moment.js our team should rely on during implementation.
• At what time length do we go from a days/months to the full date. Do we always show the year than or only if past the current year?
• How much precision is allowed on a timestamp. Seconds, minutes, hours, days, months...
• Accessibility
• When written in a sentence vs standalone
• Will we support different formats: day/month/year or month/day/year

Related

• https://github.com/github/special-projects/issues/1007

[maximedegreve]

Related: #299

[maximedegreve]

Closing as sufficient docs are now available:
https://primer.style/design/ui-patterns/dates-and-times