Update Token explanation in README

[mgrobaker]

Main changes

• Added a new section for GitHub Token setup - moved this out of Permissions section, as it seemed to deserve its own section
• Added explanation around personal access tokens. Specifically I wanted to call out that you can't use a fine-grained token. This was an issue I ran into, and I wanted to help others so they didn't encounter the same limitation without knowing about it

Minor changes

• Ran ### Repository Permissions section through Claude to make the wording flow more nicely
• Reordered screenshots. I think the the "options" part is more interesting to see first, for new users