docs: add hex grid documentation for 2D and 3D (#10)

* docs: add hex grid documentation for 2D and 3D

Comprehensive guides covering:
- Orientation (pointy/flat top) with visual diagrams
- Coordinate systems and offset explanation
- Adjacency and pathfinding patterns
- Elevation and terrain patterns (3D)
- Instanced rendering (3D)
- Complete game examples (strategy maps)
- Full API surface coverage for all hex layout functions

* fix: correct code examples in hex grid documentation

- Fix floorHex arg order (row/layer swapped) in house, plateau, rampX, civMap
- Fix terrainFromHeightmap: add maxHeight param, add missing ignore
- Fix plateau: use correct param names (surface/side not surfaceCell/sideCell)
- Replace non-existent hexRing with border in civMap example
- Add missing System.Numerics import in 3d hex examples
- Add missing System.Collections.Generic import in 2d hex pathfinding example

Author: AngelMunoz

CHANGELOG.md

@@ -12,6 +12,7 @@
 - `LayeredHexGrid3D<'T>` — Layered variant with `Dictionary<int, HexGrid3D<'T>>` layers and `LayeredHexLayout3D.layer` for composable per-layer DSL.
 - `HexGrid3DRenderer` — Rendering functions for hex grids: `render`, `renderVolume`, `renderWithIndices`, `renderInstanced`, `renderVolumeInstanced`.
 - Non-uniform dimension tests for 2D, Hex2D, and 3D grids validating correct face/edge positions for shell, border, corners, scatterShell, and scatterBorder functions.
+- Hex grid documentation: comprehensive guides for 2D and 3D hex grids covering orientation, coordinates, adjacency, pathfinding, elevation patterns, instanced rendering, and complete game examples (strategy maps, Civilization-style maps).
 
 ## [1.1.0] - 2026-06-01