tensor documentation

Reference issue

What does this implement/fix?

Added some documentation to the tensor readme. It turned out to be a lot of changes so maybe it's easiest to read the markdown rendered one directly:

https://gitlab.com/henric.ryden/eigen/-/blob/tensor_documentation/unsupported/Eigen/CXX11/src/Tensor/README.md

Additional information

Edited by Henric Ryden

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