Create new content for Designing section

[aubrey-oneal]

This section should be in alignment with Carbon Core and our other PALS, such as Cloud and dotcom.

Integrate the C4IP specifications content into the about section

Requirements:

• Create new submenu titled 'Designing'
• Create a page called 'Get started'
• Create 'Get started' guidance that is specific to this PAL, and set folks to the Core site where needed
• Move 'Design Kits' Page here
• Update 'Design Kits' Figma tab
• Deprecate Sketch tab