revisit "spectral indices" API

in #255 a new feature was added to easily calculate (multiple) spectral indices on a given data cube (EP-4053).

I think there is room to streamline this initial/experimental API:

• there are 6 very similar functions : compute_and_rescale_indices, append_and_rescale_indices, compute_indices, append_indices, compute_index, append_index I think these can be combined to a smaller set with just additional arguments/options
• the API to set up rescaling is a bit cumbersome:
  • if you discard the input data (instead of appending indices), you still have to specify input/output ranges for "collection"
  • you have to repeat the output_range, while you probably use the same for each index and input band
  • the index_dict has no explicit order of indices while this might be important for the user
• the feature needs some more documentation/examples

Start by reviewing the spectral-indices feature introduced in #255 and its current six functions, then inspect the existing documentation and examples. Clarify the intended consolidated API, rescaling behavior, and index ordering before changing anything. Done means the API is streamlined and the supported usage is documented with examples.