docs: outline of User Onboarding in User Guide

[max-cx]

What does this pull request change?

Drilling down one heading level in the User Onboarding part of the new User Guide.

1. Modifying the heading: Advanced Usage > Advanced Use
2. Splitting the User Onboarding part into two sub-parts/headings:
   i. Projects with Git
   ii. Projects without Git

What issues does this pull request fix or reference?

RHDEVDOCS-3501
RHDEVDOCS-3505

Specify the version of the product this pull request applies to

7.42

Pull Request checklist

The author and the reviewers validate the content of this pull request with the following checklist, in addition to the automated tests.

• Any procedure:
  • Successfully tested.
• Any page or link rename:
  • The page contains a redirection for the previous URL.
  • Propagate the URL change in:
    • Dashboard default branding data
    • Chectl constants.ts
• Builds on Eclipse Che hosted by Red Hat.
• the Validate language on files added or modified step reports no vale warnings.

[themr0c]

pass:[<!-- vale off -->] maybe not a good idea. The navigation is wrong in the output.