Tutorial 3 — Spin polarisation and k-points

[Chengcheng-Xiao]

Replaces the placeholder Tutorial 3 with a full tutorial covering spin-polarised (TBD) calculations and k-point sampling in ONETEP.

This PR includes:

• source/T3_spin_polarisation_and_kpoints.rst — new tutorial covering:
  • k-point sampling of the Brillouin zone (Plane-wave vs. Tight-binding modes)
  • Complex NGWFs (use_cmplx_ngwfs) and extended NGWFs (extend_ngwf)
  • Automatic (kpoint_grid_size/kpoint_grid_shift) and explicit (%block kpoints_list) k-point grids
  • k-point parallelization with num_kpars
  • Best practices (reciprocal-space NGWF initialisation, EDFT settings)
  • "Fake-kpoints" band structure workflow with kp_gen.py and plot_bands.py
  • FCC silicon as a worked example
• Supporting files under source/_static/tutorial_3/: Si.dat, Si_band.dat, Si_NCP19_PBE_OTF.usp, Silicon_Bands.png, kp_gen.py, plot_bands.py
• source/index.rst — T3 now links to the new file
• source/conf.py — updated copyright year (2024 → 2026)
  Removed: the old placeholder T3_spin_polarisation.rst.

Tagging @nickhine for a quick review and merge.