Add more specific text about what the product does.

We are planning to use the Short Description for GitLab's marketplace offerings (such as the offering on AWS Marketplace), to standardize on how we describe GitLab on each marketplace. This MR proposes some more specific text for the Short Description of GitLab to describe the available functionality so that a user browsing tools in a marketplace can quickly determine if GitLab is likely to meet their needs. This came up in this discussion about marketplace descriptions.