[Platforms] Create design spec for new side navigation

[aagonzales]

Acceptance criteria

• Create the final design spec for phase 1 (only the collapsible feature, not the popover menu)
• 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
  • SideNav Rail v2
  • SideNavigation Bar
• Review final spec with the tiger team

[aagonzales]

Spec

Figma file

Phase 1 note: there is a new sub-menu flyout behavior for the rail that will not be a part of this first release.

Grid behaviors

Side Nav - Collapsed

Side Nav - Expanded

• The Panel is a grid influencer and is persistent (remains open)
• To collapse the panel, the user must click on the "Collapse" button at the bottom of the panel.

Scrolling

When scrolling, the collapse/expand button at the bottom of the panel is sticky and items scroll underneath it. A 1px border $border-subtle is added to the top of the button when scrolled.