Render ewatercycle._forcings package on readthedocs

Currently the [ewatercycle._forcings](https://github.com/eWaterCycle/ewatercycle/tree/main/src/ewatercycle/_forcings) does not show up on https://ewatercycle.readthedocs.io/en/latest/autoapi/ewatercycle/index.html#subpackages .

This makes it impossible to see the docstrings from a browser (still works from editor/notebook/repl).

It would be nice if the _forcings module could be made public.