chore(nbs): drop ubcpdk, use generic PDK

[vvahidd]

Intent / rationale

• Remove the UBC PDK dependency from MEEP example notebooks and the docs CI workflow; notebooks now load pcells from the gdsfactory generic PDK (gpdk) or the gdsfactoryplus active PDK.

Changes introduced

• meep_ybranch/meep_crossing/meep_dc/meep_ring_coupler: load pcells via gfp.get_pdk().activate() + gf.get_component() instead of ubcpdk cells
• _meep_api_styles: load gf.components.mmi1x2() under gf.gpdk
• Delete _meep_straight.ipynb and _meep_sbend.ipynb (UBC PDK examples)
• docs.yml: remove ubcpdk pip-install case (no longer required by any published notebook)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.92%. Comparing base (c3f26a4) to head (f7616d0).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
+ Coverage   59.98%   60.92%   +0.94%     
==========================================
  Files          94       97       +3     
  Lines       13334    14101     +767     
  Branches     2703     2859     +156     
==========================================
+ Hits         7998     8591     +593     
- Misses       4439     4589     +150     
- Partials      897      921      +24     

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