docs: add CSG and frustum culling documentation with diagrams

docs: add CSG and frustum culling documentation with diagrams

Add comprehensive documentation pages for Constructive Solid Geometry
and view frustum culling, both with inline SVG diagrams explaining the
concepts visually:

- CSG page covers boolean operations (subtract, union, intersect) with
  diagrams showing input shapes and results, plus BSP tree algorithm
  explanation and polygon clipping visualization

- Frustum culling page explains the six frustum planes, AABB
  intersection testing with P-vertex optimization, and scene design
  recommendations

Reorganize main index with dedicated sections for each topic and add
inline preview diagrams. Move rendering-loop into its own directory
for consistent structure. Apply italic formatting for project name
throughout.