Add GitHub Action job summaries to workflows

[desrosj]

In May, GitHub released ​Job Summaries for GitHub Actions, which allows markdown or raw HTML to be displayed on the workflow run summary page for jobs.

This ticket is for exploring ways to use this feature in the WordPress Core workflows to make it easier to understand the data and outcomes buried in each step's log output within a job.

[desrosj]

Was chatting with @johnbillion a bit in Slack about how this feature could be utilized in Core. He ​shared one example from a project of his.

After some discussion, the most apparent place that this could be useful is within the PHPUnit workflow to display a table with data around how many:

• tests were identified as risky.
• tests were skipped.
• tests encountered errors.
• total tests were found.
• assertions were made.
• etc.

I'm marking this as good-first-bug as it's self-contained, and a great way to dig in to learn more about GitHub Actions.

Trac ticket: https://core.trac.wordpress.org/ticket/56150

[desrosj]

I think it would be great to use this feature as a way to surface the results of the performance testing introduced in [55459].

The data is submitted to an external dashboard and is visible within the logs, but having it presented as a job summary would be really neat. cc/@joemcgill @flixos90 @mukesh27

[joemcgill]

I dig this idea. Thanks @desrosj!