Add documentation for the new model docstring standard

[ablaom]

Context: #901.

Friendlier view of the addition in this PR: https://github.com/alan-turing-institute/MLJ.jl/blob/standardize-docstrings/docs/src/adding_models_for_general_use.md#document-strings

Still to do:

• A second exemplar for transformers

[codecov-commenter]

Codecov Report

Merging #906 (a9e02be) into dev (cc3ec00) will not change coverage.
The diff coverage is n/a.

❗ Current head a9e02be differs from pull request most recent head d1e2870. Consider uploading reports for the commit d1e2870 to get more accurate results

@@           Coverage Diff           @@
##              dev     #906   +/-   ##
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  Coverage   42.85%   42.85%           
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  Files           1        1           
  Lines          28       28           
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  Hits           12       12           
  Misses         16       16           

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