[reference] 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.

[bmorelli25]

I see a few missing. Maybe the conflict has something to do with it?

1. https://jolokia.org/reference/html/agents.md 🫱 https://github.com/search?q=repo%3Aelastic%2Fdocs-content%20https%3A%2F%2Fjolokia.org%2Freference%2Fhtml%2Fagents.md&type=code
2. rabbitmq.com 🫱 https://github.com/search?q=repo%3Aelastic%2Fdocs-content+http%3A%2F%2Fwww.rabbitmq.com&type=code
3. https://uwsgi-docs.readthedocs.io/en/latest/StatsServer.md 🫱 https://github.com/search?q=repo%3Aelastic%2Fdocs-content+https%3A%2F%2Fuwsgi-docs.readthedocs.io%2Fen%2Flatest%2FStatsServer.md&type=code

[leemthompo]

lgtm