update docs

[jpn--]

This pull request updates the documentation in docs/walkthrough/one-dim.ipynb to clarify the structure and relationships of the data tree and datasets. The changes improve the accuracy and depth of explanations, particularly around how dataset dimensions and relationships are defined and mapped.

Documentation improvements:

• Clarified that the data tree nodes reference dataset dimensions (not just datasets) and that the tree is only "roughly" mathematical.
• Corrected a typo in the dataset names, changing the duplicate odt_skims to odt_skims and dot_skims, and expanded the explanation of how relationships are defined and mapped between datasets, including detailed examples for multi-dimensional datasets and mapping strategies.
• Fixed a typo in the word "constructor" and improved explanation of how DataFrames are converted to one-dimensional datasets.
• Enhanced the description of mapping by label and position, and clarified special cases for categorical variables in relationship mappings.