Add overview and use cases to all EE features

We've started a new initiative to add an overview with:

# Feature name

> Introduced in GitLab 9.x.

One line short description.

## Overview

This is where the **What is this feature about** section will be.
It will describe what the feature is about at a high level,
underlining its general purpose.

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.

This is the section where specific workflows (use cases) for the
feature are described. We might not be able to think of all use
cases and we are only listing a handful, so we need to be extra
careful not to fend off potential users that their use case does
not match the documentation. Maybe an overreaction, but it doesn't
hurt to have some sort of disclaimer that the use cases we are
describing are not the only ones.

We should add this to not only the new features, but to all EE features in our documentation.

We should list all missing features and add this information. For most features, you can find a starter on gitlab-com/www-gitlab-com.

EE features task list

For Axil

For Marcia

Old ones:

All the old MRs were added by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2236

Edited by Marcia Ramos