Add doc warning for risky plugin names (related to #1694)

[somsomin]

This PR resolves Issue #1694 by adding a warning for plugin folders named lib, test, or utils. These names may cause plugin loading conflicts due to Python module resolution or backend-specific files (e.g., Slack's lib.py).

I reproduced the issue by creating a plugin in a lib/ folder, which failed to load silently. I added a log.warning(...) message in plugin_manager.py to alert developers early. I also added a documentation warning in docs/plugindevelopment.rst to guide plugin authors.

Let me know if you'd like anything adjusted.

[nzlosh]

If you intended to push code changes to this PR, you should adjust the title as it no longer only addresses documentation.