Add driver usage docs

What does this MR do?

Add documentation on how to install and setup the Fargate driver.

Why was this MR needed?

This is necessary to fully address the issue gitlab-org/gitlab-runner#2972 (closed).

Are there points in the code the reviewer needs to double check?

Please look for typos in the commands and config files, they could eventually decrease user experience (copy and paste).

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug - N/A
• In case of conflicts with master - no conflicts

What are the relevant issue numbers?

gitlab-org/gitlab-runner#2972 (closed)