Add more information and refactor format of topic

What does this MR do?

While looking at the Applications API doc, I noticed we didn't really say what we meant by Applications.

So this establishes a link the main material, and uplifts the markdown to more current styles.

Related issues

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/10421.

Author's checklist

• Apply the correct labels and milestone

Review checklist

• Your team's review (required)
• Technical writer's review (required)