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