Add exercise to Platform Engineering Chapter, TechDocs section, for publishing TechDocs to a cloud backend

[SloaneTribble]

Provide an exercise instructing students to write a GitHub Actions workflow that publishes a TechDocs site to a cloud backend of their choice. Their Backstage instance should then be able to pull documentation from the backend and display it in the UI.

The exercise itself should be fairly straightforward in that students can follow existing guides on the Backstage website.

https://backstage.io/docs/features/techdocs/configuring-ci-cd