fix path pattern in documentation pipeline

[dklimpel]

Checklist

• make test-all (UNIX) passes. CI will also test this
• unit and/or integration tests are included (if applicable)
• documentation is changed or added (if applicable)

Description of change

IMHO there is an error in the path pattern in the documentation pipeline.
The pipeline should run at any changes in docs folder. But the PR #917 and #918 show that is not the case.
But there should be a preview, especially for changes to the requirements.txt.

I was also able to reproduce this in a test.

Docs:

• https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-including-and-excluding-paths
• https://github.com/readthedocs/actions/tree/v1/preview#how-to-use-it

After a merge #917 and #918 should be rebased.