📚 Update quickstart guide to use Project API

[s-weigand]

Version information

pyglotaran = 0.6.0.dev

Documentation Section

Describe the issue

Since the Project API is supposed to be THE interaction point for users, it should also be documented in this way.
This is a follow up to #869

Expected documentation

Quickstart should use the project API, that way we also wouldn't need to always look up #869 to see how it is used. 😝

Additional context

For this it might also be a nice that Project.import_data would accept xr.Dataset and xr.DataArray, which would be also helpful for users who loaded data from a format that isn't supported by plugins.