Add action for checking documentation coverage

[stes]

• Properly pass the output. Reference: https://trstringer.com/github-actions-multiline-strings/ or even better Feature request: create wrapper to expose stdout and stderr as outputs  terraform-compliance/github_action#7 (comment)
• Use continue on error to let the documentation test fail but keep moving with running the test

[stes]

WIP. This PR checks doc coverage (but could also report other things) and then generates & posts a report. I think it would be great to get this ready to automatically test all tasks, with actionable outputs for the authors.

[github-actions]

Docstring Coverage Report

Name	Total	Miss	Cover	Cover%
base.py	7	0	7	100%
TOTAL	7	0	7	100.0%

RESULT: PASSED (minimum: 0.0%, actual: 100.0%)

[zimmerrol]

Once #86 is merged, we should update this PR and can then merge it, too.