feat(CollectionPages): Add collections
Purpose
Add collection pages for high-level categories, similar to what already exists for components. The audience is primarily the design system team.
These pages are intentionally not revealed in the navigation and only serve as a "collection" of the content pages within a certain section. This enables us to have links from the homepage, and elsewhere that can reference a set of content without needing to provide an overview page. For example, see this draft MR (!4012) that has lead-ins linking to a page like these. On the note of "overview," this concept is still used and exposed in the navigation when it's helpful for decision making or to provide higher-level context, whereas collection pages are only a group of related links with descriptions.
Specifically this MR:
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A larger number of small, related changes to keep context.
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Creates several "collection" pages for content categories:
/components/content/data-visualization/objects/patterns/product-foundations/usability
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Removes the component overview page from the navigation and changes it to a collection (links to this page have been updated too). Also removed use of
h3in the list sinceh2is skipped, and these aren't helpful as sections demarcated by headings.Before After 
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Adds descriptions for pages that don't have them. Note that no page within the Foundations or Objects section has a description, so to reduce this MR at least a little bit, descriptions have not been added there.
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Removes one page (Internationalization) that's been empty for some time with no current plans for completion.
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Updates the
/get-started/navigating-pajamaspage to list content in the same order as the nav, only include product sections of the design system, match content with the collection pages, and link to the collection pages (and overview page for data visualization).
Example collection page with descriptions
The best way to access all of the collection pages is from the updated /get-started/navigating-pajamas page.