Add a test to document how to use the folder & prefix matching

[mwouts]

Document how to use the folder & prefix matching for #974

[codecov[bot]]

Codecov Report

Merging #985 (3c8df4c) into main (12964ef) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #985   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files         118      119    +1     
  Lines       11021    11028    +7     
=======================================
+ Hits        10918    10925    +7     
  Misses        103      103           

Impacted Files	Coverage Δ
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_jupytext_nbconvert_round_trip.py	100.00% <100.00%> (ø)
tests/test_sample_notebooks_are_normalized.py	100.00% <100.00%> (ø)
tests/utils.py	100.00% <100.00%> (ø)

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