[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

```diff @@ Coverage Diff @@ ## master #1815 +/- ## ========================================== + Coverage 87.92% 88.04% +0.11% ========================================== Files 16 16 Lines 1375 1380 +5 ========================================== + 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.