[Minimal example] Add more documentation

[JonasHelming]

From: eclipse-theia/generator-theia-extension#111

Given that the extension is one of the more complex templates we have as part of the repository, we might want to document the code more, the example is likely not documented well enough for adopters to use as is.

[planger]

Yeah I agree. We should add more documentation to the code, but I'd do this after we updated the example for the next release.

[planger]

We've added a getting started section and several sections to the GLSP documentation.