Updated left nav to be workflow-based

This MR updates a section of docs.gitlab.com's left navigation to be based on GitLab workflow.

The left nav has grown organically in an "Everyone can contribute" model, and is not currently organized in a way that helps new users ramp up. Part of the nav is organized by type of user, but not all of it. Some of it is in alphabetical order, but not all of it. From a survey of doc users, we learned that they often visit multiple pages and read lots of text before finding the content they’re looking for. Many, many users tell us they resort to Google searches to find information in the docs.

Based on the results of usability testing of the left nav, we have reorganized many feature topics into a "Use GitLab" section that provides an overview of the workflow process. Our baseline study showed that part of the difficulty of finding content in docs.gitlab.com is identifying the right subsection within a category. We reorganized the nav based on workflow buckets, and retested.

The overall success rate for finding information improved, but not all reorganized areas performed better. Even though the overall success rate was low, we found that certain parts of the proposed structure performed very well. Considering the overall goal of helping users learn to use GitLab, we believe the proposed structure can move forward with a few adjustments: three sections tested lower enough to either move back to their original location or to rename.

Because of the nature of the product, some topics can seem to belong in more than one category. Our biggest outstanding question is whether Repositories and Merge Request sections belong under Build your application.

@gl-docsteam Please take a look at the Use GitLab section in the review app. Some of the left nav items are not clickable, so use the carats to navigate around. Thoughts?