workflows: fix doc generation check

[suvayu]

GH runs custom steps as bash -e (errexit), which means any command failure leads to the shell terminating. To simplify this complexity, factor out to a dedicated script, which we can now also check with shellcheck.

TestGeneratedPkgs on my fork :white_check_mark:

Note: shellcheck says the last line could be if ! cmd, but for this script that would be rather cumbersome, so I ignored it. (SC2181)

Related issues

Closes #322

Checklist

• [x] I am following the contributing guidelines
• [x] Tests are passing
• [x] Lint workflow is passing
• [x] Docs were updated and workflow is passing
• [ ] CHANGELOG.md was updated

[abelsiqueira]

@allcontributors please add @suvayu for code

[allcontributors[bot]]

@abelsiqueira

I've put up a pull request to add @suvayu! :tada: