Validate overview.md doc to ensure links actually pull content from the ditap repo instead of EdX

[bobschmitt-civicactions]

To Do:

1. Test each link in the overview.md document to ensure they pull the correct file.
2. Modify overview.md as needed and update repo.

QA Steps:

1. Login to repo
2. Go to https://github.com/usds/ditap-curriculum-update/blob/main/curriculum/overviewandmap.md
3. Click on links to see if they download material from the repo (you'll need to view the urls as things download, and make sure they aren't coming from the old DITAP site)

Note that links to live courses are still there, but need to be addressed in another ticket.

[bobschmitt-civicactions]

Program Design validated.

[bobschmitt-civicactions]

In Class sessions validated and corrected with #333

[bobschmitt-civicactions]

Additional Facilitation Material validated.

With this, all files in the EdX site have been transferred to the repo and links checks. Passing this on to @farooqzakhilwal and @april-cruz for further validation here: https://github.com/usds/ditap-curriculum-update/blob/main/curriculum/overviewandmap.md

Follow the QA steps above.