Improve External package and Nuget usage manual

[Doprez]

I would like to go through and update the Nuget export manual as Joreyk has done a lot of work to make this more stable.

Things in my mind to mention in the manual:

• Nugets require the module initializer class
• External project references outside of the solution path require the module initializer class

I also need to figure out how to properly include assets and shaders with Nugets as it seems like they are not properly imported based on stride3d/stride-community-toolkit#132 I still cant seem to get this to work in nugets. This does work though for external projects.

[VaclavElias]

That would be great. It would be also nice to mention the scenarios and benefits using this approach, so one knows why to use it, or when to use it.