Add a chapter on configuring Dask for use with ESMValTool #323

Short description of the material
Add a chapter on configuring Dask for use with ESMValTool. Current documentation: https://docs.esmvaltool.org/projects/ESMValCore/en/latest/quickstart/configure.html#dask-configuration

Branch and pull request
Once you've started working, add the branch (and pull request) link.