Rework the docs and README after renaming datasets-server -> dataset-viewer

[Sylvain Lesage]

Now that we call the project "Dataset viewer", we should make it clear in the docs and READMEs that:

• that the project powers the Hub's dataset viewer
  • people can file issues if something is broken in the Hub dataset viewer
  • contributions here will help improving the dataset viewer
• that it does not contain source code for the viewer frontend