Add Gutenberg Guidelines blueprint configuration

[smithjw1]

Summary

Adds a Blueprint that enables the gutenberg-guidelines experiment from the
Gutenberg Experiments page and drops the user directly on the Guidelines admin
screen (/wp-admin/admin.php?page=guidelines-wp-admin).

What it does

1. Installs the Gutenberg plugin
2. Runs a PHP snippet to toggle on the gutenberg-guidelines key in the
   gutenberg-experiments option
3. Lands the logged-in user on the Guidelines page so they can immediately
   interact with the feature

Testing

Open the Blueprint in WordPress Playground and confirm:

• The Gutenberg plugin is active
• Gutenberg → Experiments shows the Guidelines experiment checked
• The landing page is /wp-admin/admin.php?page=guidelines-wp-admin