Update getting_started_part_three.md

Open username-removed-421076 requested to merge xji/gitlab-ee:patch-3 into master

What does this MR do?

The tutorial on Gitlab Pages missed some critical information: You have to first add new (HTTP) domains before you can access the site or start the TLS verification process..... Otherwise the CA won't be able to find the site at the designated HTTP address! This tutorial seems to have missed that, which caused me a lot of headache

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The original tutorial just seemed to be incomplete and misleading

I intend to raise awareness of the issue. You may not approve the exact wording of the changes I propose, but I hope you do fix the tutorial as soon as possible, one way or another.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?