Simplify README.md and move detailed docs to user guides

[harrism]

In #1552 it is noted that the level of detail in README.md is quite high and growing. We should consider moving a lot of the detailed usage instructions for both C++ and Python APIs to the corresponding user guides.

Original comment:

My only question is whether README.md is getting too big. Maybe this belongs in the Python docs instead? e.g. guide.md?

Arguably yes, a question then arises as to whether we want to do a split properly. Right now, some of the information in README is replicated in the python guide. AFAICT, the README is the "canonical" source of user documentation for the C++ API, I don't know if we would rather incorporate it into the doxygen docs.

Originally posted by @wence- in #1552 (comment)