docs: add a section to Designer resources for extensions

[janchild]

Adding a new section to Other resources for kits covering different kits that we have to offer, including text toolbar, Walk me etc.

Add a line that some of these resources are only available to IBMers.

Designing > Other resources

Introduction

Elements (was Color, grid, and icons)

• leave as is - but move UI Shell template to Extensions

Extensions

• Carbon website theme Sketch library
• Data viz master Sketch file (Repeat from Data viz > Get started) -- https://github.com/carbon-design-system/carbon-design-kit/tree/master/Carbon%20Design%20Kit%20-%2010.10.0/data-visualization
• Mobile library -- https://www.sketch.com/s/a3343128-adb6-489c-9e62-709d89ba76e9
• Text toolbar library
• UI Shell template -- https://www.sketch.com/s/6a8e1d7b-f00a-4d8d-9d83-79ecf4dc12a0
• Walk me Sketch library

Fonts

GitHub repos

[janchild]

@laurenmrice and @jeanservaas, I created a draft PR and have included links to the resources that I had. Still need to know where to link to for the following:

• Carbon website theme Sketch library
• Text toolbar library
• Walk me Sketch library