Remove libGDSII dependency; demonstrate gdstk in the tutorials

[oskooi]

Closes #3206.

Context

libGDSII is an optional Meep dependency used to import GDSII layout files (polygons → Meep prisms / volumes). It is no longer maintained. This change removes libGDSII entirely from the Meep codebase — the build hooks, the C++ implementation, the SWIG/Python wrappers, the GDS tests, and the build documentation that referenced it. The Meep-native GDS functions that depended on libGDSII (get_GDSII_prisms, GDSII_vol, GDSII_layers, with_libGDSII, set_geometry_from_GDSII, get_GDSII_prism, get_GDSII_volume, get_GDSII_layers) are therefore removed as well.

Rather than wiring a replacement library into Meep's build, the gdstk Python package is demonstrated directly in the Python tutorials: users read GDS polygons with gdstk and build mp.Prism / mp.Volume objects themselves. gdstk is not added to Meep's build, requirements, or CI — it is a user-side tool shown in the docs.

Additionally, the GDS-import paths in the Python tests are deleted (analytic-prism coverage remains), and CI only has its libGDSII steps removed (gdstk is not added to CI).

Scope decisions

• gdstk is not integrated into Meep's C++/build/CI — tutorials only.
• Python test GDS paths are removed, not rewritten with gdstk.
• get_center_and_size (generaly utility, used by test_simulation.py) is kept.
• coupler.gds example asset is kept (the rewritten tutorial reads it via gdstk); python/tests/data/bend-flux.gds and python/tests/data/spiral.gds are removed.

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[stevengj]

I don't want to break working code. So:

1. Keep GDSII in the code. (However, it can print a deprecation notice when it is called)
2. Update the tutorials and docs to use gdstk
3. Keep GDSII in the installation manual but note that it is a deprecated feature that will be removed at some point.