Simplify README

[joshuadavidthomas]

Since the docs have finally been setup, the README should be pared back to just a brief intro to the package. All the details currently there should be moved to the docs for further in depth reading.

[joshuadavidthomas]

Structure

1. Intro
2. Requirements
3. Installation
4. Getting Started
   • Small example with just the basics of the API
   • One NavItem and one NavGroup with no url and one NavItem within it
   • No permissions or extra_context
   • Basic template, no styling
5. Examples
6. Documentation
7. License

[jefftriplett]

Just noting "Getting Started" from the README should probably be mirrored https://django-simple-nav.westervelt.dev/en/latest/getting-started.html