Add shaders.md with 18 stub sections across 6 parts (GPU model,
wgpu setup, vertex/fragment shaders, textures, compute shaders,
going further). Create beans tickets edu-5g0l through edu-7m8d
for each section. Add # Graphics section to SUMMARY.md.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>