Document the Qt Quick 3D mesh file format

The .mesh format that balsam writes and Model::source loads has never
been written down, so the reader was the only description of it. This
adds an internal page covering the container, the per version layout for
versions 3 to 7, and the enumerations.

Two traps get their own sections: most of the fields with "offset" in
the name are parser state that leaked into the file and have to be read
and discarded, and the alignment step adds four bytes even when the
position is already aligned.

Derived from https://github.com/nezticle/python-quick3d-mesh and checked
against the reader and the 114 .mesh files here, which between them
cover every supported version.

Change-Id: I37d2295c2e27c84ee839c5c05e4e25196d4472fe
Reviewed-by: Christian Strømme <christian.stromme@qt.io>