Document how to access CI test output artifacts

[ptitzler]

Is your feature request related to a problem? Please describe.
Sometimes CI tests fail, producing artifacts that are not included in the build log. We should update the dev documentation to document how to access those artifacts.

[ptitzler]

• open https://github.com/elyra-ai/elyra/actions

  

• locate the failing workflow

• select the workflow

• select "Summary"
  

• locate the 'Artifacts' section
  

• download the ZIP file