chore: import upstream snapshot with attribution

docs/md/explanation/table.md

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+# Table
+
+`Table` is Perspective's columnar data frame, analogous to a Pandas `DataFrame`
+or Apache Arrow, supporting append & in-place updates, removal by index, and
+update notifications.
+
+A `Table` contains columns, each of which have a unique name, are strongly and
+consistently typed, and contains rows of data conforming to the column's type.
+Each column in a `Table` must have the same number of rows, though not every row
+must contain data; null-values are used to indicate missing values in the
+dataset. The schema of a `Table` is _immutable after creation_, which means the
+column names and data types cannot be changed after the `Table` has been
+created. Columns cannot be added or deleted after creation either, but a `View`
+can be used to select an arbitrary set of columns from the `Table`.