[obj] add page about the obj file format

site-src/obj.wtn

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+* The OBJ file format
+
+OBJ is a file format for 3D models. It is line based with every line
+starting with an identifier for what the line describes. A file looks like this:
+
+``
+# comment
+o name of object
+
+# list of vertices
+v 1.000000 1.000000 -1.000000
+...
+
+# list of texture coordinates (0.0 to 1.0)
+vt 0.875000 0.500000
+...
+
+# list of vertex normals
+vn 0.0000 1.0000 0.0000
+...
+
+# face elements. these consist of indices of the previous groups. this face
+# element consists of the fifth, third, and first vertex (first number), and
+# these vertices are coupled with the first, second, and third texture
+# coordinate (second number) as well as the first vertex normal (third
+# number). note that these indices start on 1, not 0.
+f 5/1/1 3/2/1 1/3/1
+...
+``