modifier API documentation not included with sphinx-generated API documentation

[gerritholl]

Describe the bug

There is API documentation in the satpy.modifier package, but this documentation is not included in readthedocs satpy subpackages, which only includes composites, dataset, demo, enhancements, readers, and writers.

To Reproduce

• Browse to https://satpy.readthedocs.io/en/stable/api/satpy.html#subpackages
• Look for satpy.modifiers API documentation

Expected behavior

Find satpy.modifiers API documentation.

Actual results

Not find satpy.modifiers API documentation.

Additional context

It seems all other packages are present.

[djhoese]

This should be closed now with #1543. Waiting on RTD to build the docs.

[djhoese]

https://satpy.readthedocs.io/en/latest/api/satpy.modifiers.html