Document attrs.filters include and exclude helpers

[feiiiiii5]

Summary

Fixes #1627.

The attrs.filters API section now demonstrates both filtering helpers with runnable examples that cover attrs.asdict() and attrs.astuple().

Pull Request Checklist

• I acknowledge this project's AI policy.
• This pull request is not from my main branch.
• I've added tests (in tests/) which would fail without the patch (not applicable to this documentation-only change).
• I've updated the documentation (in docs/), in case of behavior changes or new features.
• I've added a new changelog entry (in docs/versionhistory.rst) (not applicable to this documentation-only change).

Testing

• Directly executed both documented examples against the worktree: both produced the expected values.
• pre-commit run --files docs/api.rst — all applicable hooks passed.
• uv run tox run -e docs-doctests was attempted twice, but dependency installation did not complete because downloads repeatedly timed out; the Sphinx doctest result is therefore not claimed.