update documentation

[koebel]

I've made some minor updates in the documentation and added more links to platforms where one can download 3D models.

Please check in particular if the url for accessing the app when running it with Docker is correct.

[saw-jan]

this would be wrong here because this section doesn't say about serving ocis.
What can be done here is, remove docker run example and only say which port does the image exposes. like this:

- Run the extension container with:
- ```bash
- docker run --rm -p3000:80 sawjan/web-app-3dmodel-viewer
- ```
+ Exposed port: `80`

This should prevent running docker run command.
Or, we can move this whole section to the buttom after Build and Run

[koebel]

I like that idea of changing the order. I think that makes it clearer. And I guess for those who use Docker frequently, I guess the would see from the command that it is exposed at port 80