Move markdown link check to run daily and Slack message failures

[rootulp]

Problem

We currently have a markdown link checker that runs in CI. It's not a required check because the check can be flaky. Unfortunately, since it's not a required check, a PR can be merged that adds a broken link and then CI for all other PRs have red ❌ s when they're entirely unrelated to the broken link.

Proposal

Move the markdown link check CI workflow from running on every PR to running daily. If the daily workflow fails, send a Slack message to a new Slack room about it. Add the #core-app team to the new Slack room.

[optional] if the workflow fails, retry it 3 times before actually sending a Slack message about it to avoid false positives.