Reorganize and create new CI Variables documentation

The current intro is too short and jumps right into a reference https://docs.gitlab.com/ee/ci/variables/

We need:

• a proper introduction and explanation of capabilities, use cases, benefits, concepts (and possibly to move the reference table to its own page)
• a tutorial doc to show an example use from A to Z (Marcia: I'm planning to give examples in-context; let's see if we need a dedicated tutorial after the MVC?)
• Consider whether to cover this item around docker-compose as part of this issue or to create a new one. (Marcia: prob out of the scope here. Will create another issue.)

Type of issue

Problem to solve

https://docs.gitlab.com/ee/ci/variables/ offers too little context, mainly a reference table

Further details

Per feedback from Mark P in this doc

Proposal

Who can address the issue

Other links/references