Improve Extension Getting Started Guide

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

rancher / dashboard