Update Getting started parent page

What does this MR do and why?

This MR updates the Getting started parent page to include references to the child topics and other relevant topics.

Changes in this MR

• Add steps to Install Git.
• Add links to relevant pages/topics.
• Add simple tasks/steps to the Practice using Git section.

Related issues

• Issue: #499459
• Child item: #499463 (closed)
• MR to update navigation: gitlab-docs!5135 (merged)