Improve Gitlab Pages documentation and requirements in regards to the `public` folder requirement

Today I spent quite a few hours trying to make Gitlab Pages work only to find out it wasn't working because I was not using a folder called public.

1. Why can't we use any other folder name?
2. Why do we need to specify an artifact with public path in the pages job if that's not configurable? The same way the pipeline automatically adds a deploy:pages job, it could add that artifact description for the pages job. By adding it by configuration, it looks configurable, but it's not.
3. Why none of the jobs fails if there's not a public folder?
4. I think the troubleshooting should have a link to Gitlab Pages requirements https://docs.gitlab.com/ee/user/project/pages/introduction.html#troubleshooting