[explore and analyze] Fix external links

[colleenmcginnis]

In the migration tool, external URLs that contained .html were incorrectly modified to use .md.

To (attempt) to fix these links:

1. I ran a script to find URLs that started with http (i.e. is not an internal or cross-repo link) and included .md somewhere in the URL.
2. Then I ran all the resulting URLs through an external link-checker.
   i. If I got a 200 status code back, I didn't do anything to the link.
   ii. If I got a 404 status code back, I swapped out the .md with .html.
3. I reran the link-checker to make sure all the updated links were now returning 200.

[florent-leborgne]

Thanks @colleenmcginnis, completely forgot to keep checking those after we first found it ❤️