Ensure no warnings in documentation builds

[AyodeAwe]

Is your feature request related to a problem? Please describe.
Currently, the cusignal documentation does not build without some issues. These issues show up as warnings in the documentation build logs. An example is here:

/opt/conda/envs/docs/lib/python3.9/site-packages/cusignal/filtering/resample.py:docstring of cusignal.filtering.resample.decimate:6: ERROR: Unexpected indentation.
/opt/conda/envs/docs/lib/python3.9/site-packages/cusignal/filtering/resample.py:docstring of cusignal.filtering.resample:3: WARNING: Block quote ends without a blank line; unexpected unindent.

This was discovered when attempting to run the sphinx-build command with the -W flag. The -W flag causes the build to fail in the event of warnings by turning warnings to errors. (See here)

Describe the solution you'd like
The ideal scenario is to have the cusignal documentation build successfully without warnings. The addition of the -W flag to the sphinx build command would ensure this.

Describe alternatives you've considered
We currently run the build command without the -W flag, ignoring any warnings that get outputted.

Additional context
About 137 warnings are emitted at the time of writing this. See this example run.

[ajschmidt8]

For reference, the PR where is issue was discovered is below.

• Add docs build job #536