docs: Add ADR about typing

docs/design/0003-use-union-rather-than-pipe.md

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+---
+date: 2025-01-14
+decision-makers: ["@augustebaum", "@thomass-dev"]
+---
+
+# Use `typing.Union` rather than `|` in type hints
+
+## Context and Problem Statement
+
+This ADR originates from a discussion in [this PR comment](https://github.com/probabl-ai/skore/pull/1084#discussion_r1914402895).
+
+Writing a union type using `|` is [only available from Python 3.10](https://docs.python.org/3.12/library/typing.html#typing.Union), while we currently intend to support Python 3.9.
+Even though we do not formally check type-hints using automatic tools like `mypy`, we still strive for consistency for style issues of this kind.
+
+## Decision Outcome
+
+Use `typing.Union` rather than `|` in type hints.