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
Review checklist
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Your team's review (required) -
Technical writer's review (required)
Edited by Evan Read