Add margin to alert boxes

The alert boxes were too close to the preceding sentence and it was suffocating me 😄 I used the trick I learned from @oregand to add a bootstrap class that adds some margin! I tested with md-2 and md-4, and it seems md-3 is the better solution.

before	after

I took the above screenshots from https://docs.gitlab.com/ee/ci/yaml/README.html#ruleschanges.

If the alert is after a list, the margin works as expected https://docs.gitlab.com/ee/ci/. With that class addition nothing seems to change in that aspect, so we're good.