Initiating data visualization section

Creating a placeholder section for the Data visualization content category. Color will likely be the initial default entry now that the palette has been added to gitlab-ui, and documentation is forthcoming.

Out of scope for this MR:

• Migrating content from https://design.gitlab.com/components/data-visualization
• Defining the default content viewed when selecting Data visualization in the nav (currently will default to the first subpage, which happens to be Color).
• Reconsidering the component:data-visualization label

Reviewer

• Ensure that the new category name (Data visualization) is correct
• Validate the placement of this new category after Components and before Regions as the desired location

/cc @npost @jareko