Clear up confusion between GitHub Action and App topics

[davidwrede]

Problem description

We have some customer confusion how GitHub Actions and GitHub App differ from each other and how they can be used together.

Suggestions for a fix

• [ ] (Short-term fix) Cross-link between topics with a descriptive sentence that gives a high-level idea of the other GH topic.
• [ ] (Longer-term fix) Merge the two GH topics together under the title "Pulumi GitHub Integration". Provide high-level comparison between the two and how-to steps for using them together.