[Platforms] Create design spec for the header navigation panel

[aagonzales]

Acceptance criteria

In addition to adding the tree view dynamic to the header navigation panel, we need to determine how the header panel looks and functions. Is it temporary or persistent? Does it overlay content or is it a grid influencer? If an overlay, does it shadowbox the page content when open? Or does it need a drop-shadow like IBM Cloud global header nav?

• Create the final design spec for phase 1
• Provide interaction behaviors, accessibility considerations, color tokens, and spacing annotations in the spec
• Provide an image of the final design spec in this issue
• Determine final name for component/variant
• Review final spec with the tiger team