Remove ubcpdk dependency from example notebooks

[jackgdsf]

Example notebooks under nbs/ import ubcpdk for basic photonic components (straight, coupler, bend_s, etc.), but these all have equivalents in gdsfactory.components. Using the generic PDK removes the external dependency and keeps examples self-contained.

Components to replace:

• cells.straight → gf.components.straight
• cells.bend_s → gf.components.bend_s
• cells.coupler → gf.components.coupler
• cells.coupler_ring → gf.components.coupler_ring
• cells.ebeam_y_1550 → gf.components.mmi1x2
• cells.ebeam_crossing4 → gf.components.crossing45