Add domains that should be allowed on restrictive networks

[parkerbxyz]

We should add a list of GitHub domains that should be included in an allow list on restrictive corporate networks. This can be included on the “Getting Ready for Class” page.

• github.com for obvious reasons.
• github.io for GitHub Pages.
• githubusercontent.com for GitHub-hosted images, raw source code, etc.
• githubassets.com to load CSS and JavaScript assets on github.com.