DOC: Discuss slicer interface

[effigies]

We never documented the slicer interface outside the API docs, which is less prominent than the feature deserves, given its usefulness, IMO.

Comments appreciated.

Follow-up to #550.

[codecov]

Codecov Report

Merging #846 into master will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #846      +/-   ##
==========================================
- Coverage   90.09%   89.78%   -0.32%     
==========================================
  Files          98       98              
  Lines       12450    12450              
  Branches     2188     2188              
==========================================
- Hits        11217    11178      -39     
- Misses        883      918      +35     
- Partials      350      354       +4

Impacted Files	Coverage Δ
nibabel/pydicom_compat.py	63.63% <0%> (-36.37%)	⬇️
nibabel/nicom/dicomwrappers.py	85.16% <0%> (-5.75%)	⬇️
nibabel/__init__.py	89.28% <0%> (-3.58%)	⬇️
nibabel/viewers.py	95.49% <0%> (-0.65%)	⬇️
nibabel/nifti1.py	91.21% <0%> (-0.31%)	⬇️
nibabel/volumeutils.py	92.45% <0%> (-0.2%)	⬇️

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[effigies]

Doctest passing. Please open a PR if you have suggestions for improvements.