Docs

[nicola-bastianello]

WIP todo list for the docs (from my comments in #1 )

• illustrate how to run the benchmarks and interpret the results (especially how to run them with compiled and uncompiled versions)
• discussion about TF eager tensors being immutable, and round-trip through numpy (see tensorflow_backend)
• dev: "Its important that the enum values correspond to the folder names of the backends" in types.py
• we should probably find a way to divide the interop api in different pages, each collecting a category of functions (math, linalg, rng, etc). we can discuss the best division