docs: Add v3 migration guide

[Oliver Borchert (borchero)]

Motivation

This should finalize the changes for v3.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d1670de) to head (2b09d04).

Additional details and impacted files

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[Copilot]

Pull request overview

Adds a new v2→v3 migration guide to document the intended v3 API changes and links it from the migration guide index, aiming to finalize documentation around the v3 transition.

Changes:

• Added docs/guides/migration/v2-v3.md migration guide covering (intended) v3 improvements and breaking changes.
• Updated the migration guide toctree to include the new v2→v3 guide.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docs/guides/migration/v2-v3.md	New migration guide describing v2→v3 changes and recommended code updates.
docs/guides/migration/index.md	Adds v2-v3 to the migration guide navigation.

Comments suppressed due to low confidence (2)

docs/guides/migration/v2-v3.md:20

• The statement that dataframely no longer relies on pyarrow as an optional dependency appears incorrect: the library still conditionally imports pyarrow (via dataframely._compat.pa) and exposes Schema.to_pyarrow_schema() / Column.pyarrow_dtype code paths that require pyarrow. Consider rephrasing to avoid implying pyarrow support is removed.

As a result, dataframely does not rely on `pyarrow` as an optional dependency anymore.

docs/guides/migration/v2-v3.md:26

• This section describes a categories argument and references pl.Categories, but dataframely.Categorical does not accept a categories parameter and there is no pl.Categories usage in the codebase. The example dy.Categorical(pl.UInt16) / dy.Categorical(pl.Categories(...)) will not work. Consider documenting dy.Enum (which does accept categories and maps to pl.Enum) instead, or update the text to match the actual v3 categorical API.

### Refined `dy.Categorical`

{class}`~dataframely.Categorical` now accepts a `categories` argument, allowing to pass a `pl.Categories` object.
Alternatively, a data type may be passed, which automatically infers `name` and `namespace` of the `pl.Categories`
object as the column and schema names, respectively, scoping the categorical to the specific column of a single schema.

[Andreas Albert (AndreasAlbertQC)]

Ah, here's the guide already, nice, thanks!