`cufft.py` module excluded from API docs

In #494 the `cufft.py` module was intentionally excluded from the `index.rst` file: https://github.com/DiamondLightSource/httomo/blob/8a55c4dc62aae97d9d7f4d264602a2cfa96cd23e/docs/source/developers/api.rst?plain=1#L1-L28

due to not knowing how to handle the error thrown when the CuFFT library isn't found: https://github.com/DiamondLightSource/httomo/blob/8a55c4dc62aae97d9d7f4d264602a2cfa96cd23e/httomo/cufft.py#L29-L41

The `cufft.py` module is purely for memory estimation, and because the memory estimators will be moved from httomo to httomo-backends (see #429), this issue will likely become irrelevant to httomo and instead become relevant to httomo-backends.