Add training+certs showcase to Backstage website

[chalin]

Training+certs for Backstage will be ready in a few months. Showcase them on the website.

From @caniszczyk:

Something like https://prometheus.io/support-training/

Similar to:

• Add training+certs showcase to OTel website #197

[rolfedh]

Hi @chalin. Thanks for opening this issue. What is the status of the Backstage training and certifications? Is there a target URL where we might want this information to appear?