provide help about the ELN instance

https://github.com/aiidalab/aiidalab-eln/blob/1772dcd53856e6d9da8a8eb8225fd02ccb428713/aiidalab_eln/cheminfo/__init__.py#L30

i think this is not necessarily clear to users. Do they have to provide http? https? www? do they have to provide the trailing eln? Where do they get this info.

I think we should add a help text with something like `you can find the URL for the ELN instance in Tools -> Access Token`.

Alternatively, we can also print it in the iframe