Moved docs around a bit

doc/attributes.md → doc/concepts/attributes.md

 group of lines from "bullet list" to "number list" we need to update
 block style on each of the lines individually. Zefyr editor abstracts
 away such details with help of [heuristic rules][heuristics].
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-### Next up
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-* [Heuristics][heuristics]
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-[heuristics]: /doc/heuristics.md
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-### Previous
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-* [Data Format and Document Model][data_and_document]
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-[data_and_document]: /doc/data_and_document.md

doc/data_and_document.md → doc/concepts/data_and_document.md

 
 [heuristics]: /doc/heuristics.md
 [attributes]: /doc/attributes.md
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-### Next
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-* [Attributes](/doc/attributes.md)
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-### Previous
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-* [Usage](/doc/usage.md)

doc/heuristics.md → doc/concepts/heuristics.md

 sure to use `ChangeSource.remote` when calling `compose()`. This allows
 you to distinguish such changes from local changes made by the user
 when listening on `NotusDocument.changes` stream.
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-### Next up
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-* [Images][images]
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-[images]: /doc/images.md
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-### Previous
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-* [Style attributes][attributes]
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-[attributes]: /doc/attributes.md

doc/images.md

   }
 }
 ```
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-### Previous
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-* [Heuristics][heuristics]
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-[heuristics]: /doc/heuristics.md

doc/quick_start.md

 
 In following sections you will learn more about document
 model, Deltas, attributes and other aspects of the editor.
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-### Next
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-* [Data Format and Document Model][data_and_docs]
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-[data_and_docs]: /doc/data_and_document.md