fix: override alert shortcode for hugo rendering change

[grayside]

This change borrows a key modification from https://github.com/google/docsy/pull/941, fixing how the alert shortcode renders in versions of hugo released in the last couple years.

Once docsy ships an update with that fix, we can simply remove this override and it will fall back to wherever the theme's code lands.

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-123/

[github-actions[bot]]

🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-123/

[github-actions[bot]]

🧨 Preview deployments removed.