Documentation update and new instructions

[silviorp]

I'm trying to follow the documentation in order to make an example project with Nuvigator and I've found two problems with that:

• The documentation examples suggests that we use a GlobalRouter instance as the top level Router but looks like this class was removed in version 0.3.0.
• Since the generated code is using Extensions, I had to do some research to find out that I'd needed to create a new file in the root directory with the name analysis_options.yaml and the following content to make it to work:

  enable-experiment:
  - extension-methods

What about updating the docs with those details?

[leoiacovini]

Closing due to changes in the API and new documentation has been made