Update guide links to external-module-framework-docs

[paulj3]

Hello! I was working through the exercises and I noticed the URLs point to VUMC's old documentation before they made a git sub repo over to https://github.com/vanderbilt-redcap/external-module-framework-docs/tree/main. I made a PR with some of these changes, but I don't have push rights so I could not push it!

[pbchase]

@paulj3, I'm sorry I haven't responded to this sooner. Do you still have the branch with the changes? If you fork this repo, you should be able to push to your fork, then make a PR in this repo.

I'd love to review your work. Unfortunately, I've been neglecting this repository. It's two years overdue for an update