Create a visual identity

[chsxf]

Create a visual identity for CiderCSSKit, based on the one made for CiderKit.

• Icon
• Social images for the repository
• Updating the documentation to reflect that

Icon design reference from Apple:
https://1drv.ms/u/s!AtX-9tNTFBqfh_wYg_qgOiZnhTUUKg?e=PQJrdE