Improve CI/CD getting started documentation

Description

CI/CD Getting Started documentation (https://docs.gitlab.com/ee/ci/#getting-started) is more a collection of links to other pages, but there is no real help in "getting started". If I want to start using GitLab CI/CD without any previous knowledge, and I go there, probably I'll give up.

Proposal

Improve the documentation in order to be a real getting started guide, with steps and examples on how to start with GitLab CI/CD. Links are fine, but they should be more consistent and organized.

Links / references

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.