GitLab basics overhaul: create a project

From @rdickenson's video teardown (https://www.youtube.com/watch?v=e9lCeFQpDZs) of https://docs.gitlab.com/ee/gitlab-basics/create-project.html, improve the documentation using the following notes:

• Documentation doesn't explain what a project is, or why I want one.
• Note at the top comes a bit early.
• The four options given don't tell you why you'd choose one or the other.
• Title of sub sections should be perhaps be made verbs. For example, Creating blank projects.
• Some fields in Black projects section aren't mentioned.
• Project templates lists two types without saying why you'd pick one of the other.
• Custom project templates doesn't explicitly state what each of the types are (though it's heavily implied).
• Could move the info about creating your own Custom project templates., because they aren't about using the templates to create a project.
• Push to create a new project section comes out of the blue, it's not in the initial list of ways of creating a project.
• Instructions in Push to create a new project could be improved to:
  • Show what parts of commands need to be replaced.
  • If any other configuration is required in the UI.

Suggestion:

• Incorporate feedback above.
• Make sure all sub sections are introduced at top.

@rdickenson would you add anything?

CC @gl-docsteam