docs: Reorganize 3D.md and integrate tasks

- Moved 3D.md into the doc/ directory to centralize documentation.
- Updated README.md to reflect the new location of 3D.md.
- Captured the action items from 3D.md (visual debug mode, Blender exporter) and added them as a new task (#18) in the PROJECT_CONTEXT.md roadmap.

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+# 3D system and rendering pipeline
+
+This sub-project describe how the 3D worlds are going to be rendered.
+We want a camera to move around moving and dynamic objects.
+These objects can be need a physics and collision system.
+
+## the idea
+
+Each object has:
+  * a bounding box or capsule
+  * a BVH is maintained while these move around physically (or not)
+  * to render a frame we cull these bounding volumes
+  * at rendering time, the bounding box or sphere is turned into a quad or triangle
+  fan and a shader associated with the object is called (after proper world-object-camera transformations)
+  * each object can be queries for:
+     a) ray-object intersection ("return the distance from the object at this point P in this direction D")
+     b) Signed Distance Field ("what is the minimum distance to the object from this point P?")
+
+So in the end, we'll
+   a) move the camera and lights along paths
+   b) transform the bounding volumes and cull for visible boxes
+   c) project them
+   d) call the objects' respective shaders for rendering
+
+We want to use shadow maps, so multi-passes is expected.
+
+## debugging features
+
+The assist the visual debugging, we need a 'visual_debug' mode (code to be
+removed with STRIP_ALL) that:
+
+   a) draws a wireframe around the bounding volumes (box, etc.)
+   b) draw the trajectories (camera, objects, ...)
+   c) show the collision points
+   d) displays the ray/object intersection interactively
+   e) show the lights (direction, cone, ...), display their shadow-map in 3d and on-screen (2d).
+
+This must be captured and tracked as a sub-task
+
+
+## future step
+
+Have an exporter from Blender modelling software. That would be a converter
+from simple blender-exported files to our internal format (as an asset for
+our AssetManager or as c++ code directly)
+
+## latter improvement
+
+How to handle transparency? Multi-Ray-casting?
+We need to think about the lighting strategy.