FIX: Split migration into two steps in developer guide (PR #8103)

To temporary mitigate the problem, we need to split rake db:create and rake db:migrate into two commands.

It was discussed here: Hello from Gitpod! (installing on google cloud + automated dev setup) - installation - Discourse Meta

GitHub

You’ve signed the CLA, lis2. Thank you! This pull request is ready for review.

Thanks, seems good!