Improve Extension Getting Started Guide

[richard-cox]

https://extensions.rancher.io/extensions/next/extensions-getting-started

This needs to be converted into a clear A --> Z guide and possibly broken up

• "Creating an extension" covers creating the root app and 1 extension, this isn't very clear
• Order isn't logical
  • asked to run yarn create @rancher/extension my-app
  • then info on installing rancher (this shouldn't be embedded in this guide, if at all it should be a link outside)
  • then in the same section as installing rancher details of the options for create @rancher/extension takes
  • then instructions on how to run rancher using root app
• Creating another extension pkg
  • this starts with the same command as above, which would create the root app (does it skip if exists?)
  • not sure why his isn't covered in the section above?
• Configuring extension (shouldn't this be writing extension, it's not really config)
• Running the App
  • this repeats the instruct from above
• Loading Into Rancher
• Creating a Release
• Wrap-up