Moving documents to MkDocs.

doc/README.md

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+# How to setup documentation environment
+
+```
+$ python -m pip install mkdocs
+$ python -m pip install mkdocs-material
+$ python -m pip install markdown-katex
+```

doc/docs/index.md

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+# TinyUSDZ documentation
+
+For full documentation visit [mkdocs.org](https://www.mkdocs.org).
+
+## Mesh prim
+
+* points : usually vec3f[]
+* normals : usually vec3f[] varying
+* primvars:st : usually vec2f[] varying
+* primvars:st:indices : usually int[]  varying
+* velocities : usually vec3f[]
+* texcoord: usually `texCoord2f[]` (Texcoord bound to Mesh prim) 
+
+
+#### Mesh property
+
+* doubleSided : bool
+* extent : floa3[2]
+* facevaryingLinearInterpolation : cornerPlus1
+* faceVertexCounts : int[]
+* faceVertexIndices : int[]
+
+* orientation : ty: token. uniform. allowedTokens: ['rightHanded', 'leftHanded']
+* visibility : ty: token. varying. allowedTokens: ['inherited', 'invisible']
+* purpose : ty: token. uniform. allowedTokens: ['default', 'render', 'proxy', 'guide']
+* subdivisionScheme : ty: token. uniform. allowedTokens: ['catmullClark', 'loop', 'bilinear', 'none']
+* trianglesSubdivisionRule : ty: token. uniform. allowedTokens: ['catmullClark', 'smooth']
+ 
+* xformOpOrder : ty: token. uniform.
+
+* material:binding : Path
+* proxyPrim : ty: relation. uniform
+
+##### Optional property
+
+* primvars:displayColor : ty: float3[], varying
+  * Usually array len = 1(constant color over the mesh)
+* primvars:displayOpacity : ty: float[], varying
+  * Usually array len = 1(constant opacity over the mesh)
+
+#### SubD property
+
+* cornerIndices : int[]
+* cornerSharpnesses : float[]
+* creaseIndices : int[]
+* creaseLengths : int[]
+* creaseSharpnesses : float[]
+* holeIndices : int[]
+* interpolateBoundary : token?
+
+### Xform
+
+* world bounding box : range3d
+* local world xform : matrix4d
+* purpose
+* visibility
+* xformOpOrder
+
+### Shader
+
+* inputs:id : UsdUVTexture
+* inputs:fallback : fallback color(Usually vec4f)
+* inputs:file : @filename@
+
+#### Texcoord reader
+
+`UsdPrimvarReader_float2`
+
+* inputs:varname(token) : Name of UV coordinate primvar in Mesh primitive.
+* outputs:<name>(float2) : Output connection name. Usually `outputs:result`
+
+### TODO
+
+* [ ] non-vec3f `position`
+* [ ] vertex color
+* [ ] xformOpOrder
+* [ ] Multi texcoord UVs
+
+
+## PreviewSurface
+
+https://graphics.pixar.com/usd/docs/UsdPreviewSurface-Proposal.html 
+
+### Inputs
+
+| Name               | Type         | Default value, possible value |
+| :--                | :--          | :--                           |
+| diffuseColor       | color3f      | (0.18, 0.18, 0.18)            |
+| emissiveColor      | color3f      | (0.0, 0.0, 0.0)               |
+| roughness          | float        | 0.5                           | 
+| clearcoat          | float        | 0.0                           | 
+| clearcoatRoughness | float        | 0.01                          | 
+| opacity            | float        | 1.0                           | 
+| opacityThreshold   | float        | 0.0                           | 
+| ior                | float        | 1.5                           | 
+| normal             | normal3f     | (0.0, 0.0, 1.0)               |
+| displacement       | float        | 0.0                           |
+| occlusion          | float        | 1.0                           |
+| useSpeclarWorkflow | int          | 1(specular) or 0(metallic)    |
+
+Specular workflow
+
+| Name               | Type         | Default value   |
+| :--                | :--          | :--             |
+| specularColor      | color3f      | (0.0, 0.0, 0.0) |
+
+Metallic workflow
+
+| Name               | Type         | Default value   |
+| :--                | :--          | :--             |
+| metallic           | float        | 0.0             |
+
+
+### Outputs
+
+* surface - token
+* displacement - token
+
+### UsdUVTexture
+
+#### Inputs
+
+| Name               | Type         | Default value, possible value |
+| :--                | :--          | :--                           |
+| file               | asset        | string                        |
+| st                 | float2       | (0.0, 0.0)                    |
+| wrapS              | token        | black, clamp, repeat, mirror  |
+| wrapT              | token        | black, clamp, repeat, mirror  |
+| fallback           | float4       | (0.0, 0.0, 0.0, 1.0)          |
+| scale              | float4       | (1.0, 1.0, 1.0, 1.0)          |
+| bias               | float4       | (0.0, 0.0, 0.0, 0.0)          |
+
+#### Outputs
+
+* r, g, b, a
+
+
+### UsdTransform2d
+
+#### Inputs
+
+* in - float2 - (0.0, 0.0)
+* rotation - float - (0.0)
+  * counter-clockwise rotation in degrees around the origin
+* scale - float2 - (1.0, 1.0)
+* translation - float2 (0.0, 0.0)
+
+#### Outpuits
+
+* result - float2
+
+
+### TODO
+
+* [ ] Primvar Reader(arbitrary vertex attributes)
+* [ ] displacement
+* [ ] Texture transform
+
+
+## Notes on USDC crate file format
+
+### Bootstrap(header)
+
+* magic header: "PXR-USDC" : 8 bytes
+* version number: uint8 x 8(0 = major, 1 = minor, 2 = patch, rest unused) :8 bytes
+* int64_t tocOffset
+* int64_t _reserved[8]
+
+=> total 88 bytes
+
+### Version
+
+From pxr/usd/usd/crateFile.cpp
+
+```
+// Version history:
+// 0.9.0: Added support for the timecode and timecode[] value types.
+// 0.8.0: Added support for SdfPayloadListOp values and SdfPayload values with
+//        layer offsets.
+// 0.7.0: Array sizes written as 64 bit ints.
+```
+
+* 0.8.0(USD v20.11)
+  
+### Compression
+
+version 0.4.0 or later uses LZ4 for compression and interger compression(pxr original? compression. its backend still uses LZ4) for index arrays.
+
+### TOC(table of contents)
+
+List of Sections.
+
+### Sections
+
+There are 6 known sections.
+
+#### TOKENS
+
+List of strings(tokens). tokens are unique.
+
+#### STRINGS
+
+List of StringIndices(index to `tokens`)
+
+#### FIELDS
+
+Data field info. This is a bit tricky data format.
+See the source code for details.
+
+Field is consist of
+
+* index to token
+* 8 byte info. 2 byte for type an so on, 6 byte for primitive value or offset to the value(e.g. for array data)
+
+#### FIELDSETS
+
+List of indices
+
+#### PATHS
+
+List of path indices.
+
+Need to reconstruct
+                          
+#### SPECS
+
+List of specs
+
+###### Spec
+
+* path index
+* fieldsets index 
+* SdfSpecType

doc/mkdocs.yml

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+site_name: TinyUSDZ 
+
+theme:
+  name: material
+  palette:
+    scheme: slate
+
+markdown_extensions:
+  - markdown_katex:
+      no_inline_svg: True
+      insert_fonts_css: True
+      macro-file: macros.tex
+  - admonition