[ENH] move definition of raw and derivatives datasets to common principles

[Remi-Gau]

• closes None but relates to https://github.com/bids-standard/bids-specification/issues/1812#issuecomment-2095632418

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• HTML
  • https://bids-specification--1815.org.readthedocs.build/en/1815/common-principles.html#definitions
  • note that those definitions are now also part of the glossary

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• This makes the definition of raw and derivative both more official and easier to find.
• Add some semantic line break.

TODO

• [x] link to the definitions in the relevant derivate related sections

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.04%. Comparing base (0950f6d) to head (e26449f). Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1815      +/-   ##
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+ Coverage   87.92%   88.04%   +0.11%     
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  Files          16       16              
  Lines        1375     1380       +5     
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+ Hits         1209     1215       +6     
+ Misses        166      165       -1     

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[CPernet]

ok but will you keep the intro https://bids-specification.readthedocs.io/en/stable/derivatives/introduction.html with examples or will this go away?

[Remi-Gau]

hum true that the examples look a bit weird without the previous sentence. I will put back that sentence.