Adds default branch name instructions to Creating a new project

This change is in support of moving to main as the name of default branches in projects by adding a step to make the default branch for new projects by adding to the instruction when creating a new engineering project. main is currently the choice on https://gitlab.com/gitlab-org/gitlab/-/issues/221164 for making the change the default in the product.

References https://gitlab.com/gitlab-org/gitlab/-/issues/221164.

This MR was created from the Static Site Editor and was created without the default template.