Simplify install instructions

What does this Merge Request do?

Attempts to simplify the installation instructions for the most common use case. Takes multiple sections and moved them into a numbered list, indicating a sequence to follow.

• Before: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/index.md
• After: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/ee20ecd4970602834b03007a36fcac4c6657ba56/doc/index.md

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.