core: stdcm: add link to website documentation in the code

[eckter]

The website doc really helps but it's easy to miss, it should be referenced in the code

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.04%. Comparing base (5676323) to head (bc8938a).
Report is 2 commits behind head on dev.

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #8097      +/-   ##
============================================
- Coverage     28.05%   28.04%   -0.01%     
  Complexity     2081     2081              
============================================
  Files          1289     1289              
  Lines        157718   157727       +9     
  Branches       3114     3114              
============================================
- Hits          44251    44242       -9     
- Misses       111592   111610      +18     
  Partials       1875     1875              

Flag	Coverage Δ
core	75.19% <ø> (ø)
editoast	70.75% <ø> (-0.04%)	⬇️
front	9.90% <ø> (-0.01%)	⬇️
gateway	2.34% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	72.93% <ø> (ø)

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