Add more examples about fbxcel-dom

[lo48576]

To get raw vertex (called "control point" in FBX), you should get control point index at first.
A polygon (n-gon) is described as a list of "polygon vertex"es, which is a list of pairs of a "control point index" and polygon end marker (i.e. whether the vertex is the last one of the current n-gon).
You can get a polygon vertex from "polygon vertex index" and PolygonVertices, and yes, currently there is no way for users to get raw n-gon...!!
I completely missed that. 😭

[lo48576]

And, about merging different vertices at the same position, no it is not supported.
There are many multi-level "mapping" types from one index type to another index type (see MappingMode and ReferenceInformation).
I've thought it is very hard to deduplicate these info. Merging some "control point"s can break every other indices at any level.