Audit GitHub integration and build process guidance

[stevector]

We have a few pages/sections that touch on the use case of managing a site's code on GitHub/BitBucket/GitLab and how to use a build process in between those external repos and Pantheon. While it's not urgent, we likely need to do a clean up to provide a clearer recommendation. Build Tools has been our de facto answer for years, but GitHub Actions is growing in popularity too. I'd like to answer questions like https://community.pantheon.io/forum/t/how-to-use-github-actions-to-syncronize-code-and-compile-your-css-js-before-deployment/272 with a clear link to Docs.

Relevant links:

• https://digisavvy.com/2021/12/22/automate-synchronization-between-pantheon-and-github/
• https://www.thirdandgrove.com/insights/developing-new-pantheon-workflow-powered-github-actions/