Add doctests for seymour_decomposition

[xuluze]

The missing part is that three sum requiring a fix.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[mkoeppe]

Thanks!