Documentation improvement

1. Add CI test for building documentations (Do not ignore `warnings` and add spellcheck).
2. Fix docstrings with incorrect/inconsistent Sphinx format. Currently, such issues are treated as `warnings` in the docs building.

Start by locating the existing documentation build configuration and CI workflow, then identify how warnings are currently handled and where spellchecking can be added. Review the Python docstrings that produce Sphinx warnings and bring their formats into consistency; done means the documentation build passes without ignored warnings and includes spellcheck validation.

Written by the indexing model from the issue text.