Add better Dagster instructions to data update documentation

[marianneke]

Overview

The instructions on how to use Dagster effectively during data updates can be improved, for example showing how to materialize the necessary assets for a given dataset (e.g. raw and downstream)

Success Criteria

How will we know that we're done?

• ...

Next steps

• ...

[e-belfer]

How to manually select "all assets downstream of X" and "lower concurrency" are two things I use often and would include in here.