Implement Create Menu in super sidebar navigation

Proposal

Using <GlDisclosure>, implement the create menu in the super sidebar navigation. Clicking the + triggers the disclosure, clicking outside closes it.

Things to note

• We should replicate the funcionality of the current "create" menu
• This will be behind both the super_sidebar_nav feature flag and the navbar toggle.

Designs

See attached design files and the Navigation redesign file in figma

Please ensure the MR is reviewed by UX