CI / DOC: skip tests when only files in doc/ is changed

[jorisvandenbossche]

For a doc-only change (so inside the doc/ directory, not for docstrings), it should be possible to skip most of the tests builds in CI, and only running the linters and web/docs build should be sufficient.

This gives 1) faster CI feedback for doc PRs, 2) less chance on unrelated (flaky) failures on such PRs, and 3) in general reduces the CI load.

For the github actions, there is a on.paths option to do this, and it actually supports the "exclude paths" feature that we need: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-ignoring-paths

For the Azure builds, it seems this might also be possible: https://docs.microsoft.com/en-us/azure/devops/pipelines/repos/azure-repos-git?view=azure-devops&tabs=yaml#paths

cc @afeld