Add advanced column generation docs for HiGHS and OpenJij

[termoshtt]

Summary

• Add a new advanced example page that walks through a CVRP column generation workflow using HiGHS for the master problem and OpenJij for pricing.
• Document reusable API shapes for a generic column generation helper, including master rows, columns, pricing context, and solver/oracle callbacks.
• Add an advanced examples index and link it from the English docs landing page.

Testing

• Not run (not requested)