Move GitHub PAT instructions to beginning along with other config

[pschloss]

Need to add proper configurations to lesson materials

[kelly-sovacool]

The material on Personal Access Tokens should be moved to earlier in the lesson, e.g. just after the other config steps.

[kelly-sovacool]

Also we should probably strongly suggest that people use their OS's built-in credential helper (osxkeychain or Windows Credential Manager) for better security. See this blog post for discussion: https://usethis.r-lib.org/articles/git-credentials.html