Create docs for the <relative-time> element in advanced markdown formatting

[RiskyMH]

Why:

I recently saw the <relative-time> element in action but couldn't find much documentation for it, so thought it would be good to document it.

It seems to be mostly the same as https://github.com/github/relative-time-element#usage, just with less support.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Example usage: April 1, 2014

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
New file: get-started/writing-on-github/working-with-advanced-formatting/displaying-relative-time.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18
get-started/writing-on-github/working-with-advanced-formatting/index.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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