Add DCC'26 workshop notebook suite (participant + solutions)

[SoheylM]

Summary

This PR adds the DCC'26 workshop notebook suite to EngiOpt with participant and facilitator tracks.

What is included

• New workshops/dcc26/ folder with:
  • participant/ notebooks with guided PUBLIC FILL-IN exercises and checkpoints
  • solutions/ notebooks for facilitators
  • workshop runbook and pedagogy blueprint docs
  • helper utilities and a local requirements snapshot
• Notebook 03 scaffold upgraded to a PyBullet robotics co-design problem (PlanarManipulatorCoDesignProblem)
• Optional EngiOpt generative extension in Notebook 03 using engiopt.cgan_1d
• Colab-friendly dependency bootstrap cells across notebooks
• README updates linking workshop assets
• Git ignore updates for workshop-generated artifacts

Public API impact

• No changes to EngiBench/EngiOpt core runtime APIs.
• This PR adds tutorial/workshop assets only.

Notes for review

Please focus on:

1. participant notebook clarity and pacing
2. Colab reliability
3. documentation and public-repo hygiene

Validation

• Notebook code cells were iteratively smoke-checked and compile-checked during development.
• Notebook JSON has no execution counts or persisted outputs.