Update readme based on internal redesign

[frederikprijck]

Description

Restructures the readme file based on our internal design to try and create a uniform experience across the readme's of our different SDKs.

• [x] Add header image

Checklist

• [x] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [x] All active GitHub checks for tests, formatting, and security are passing
• [x] The correct base branch is being used, if not the default branch