Add installation steps for new plugins in blueprint

[courtneyr-dev]

Add beta-tester plugin to the releases

[github-actions]

Test using WordPress Playground

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[brandonpayton]

Discussing this in WP.org Slack:
https://wordpress.slack.com/archives/C04EWKGDJ0K/p1772201935584039?thread_ts=1771609394.946579&cid=C04EWKGDJ0K

before the release is published officially, they can test the upgrade from 6.9.1 > 7?

This blueprint targets the beta version of WP today. Does that interfere with the above goal?

Another way to test the upgrade after the release would be to use a Playground URL like this:
https://playground.wordpress.net/?plugin=wordpress-beta-tester&wp=6.9

I'm happy to merge this Blueprint update if it makes sense for testing, but it seemed good to better understand the change first.