Figure out documentation of backends #16

Right now we when a function has implementations for other backends, we add the following to the documentation:

Backends
--------
backend_name :
    <additional_docs> 

Where additional_docs is a per-function attribute (or "no backend documentation available").

NetworkX does something similar, for example see at the end here.

NetworkX goes a bit further and also has "additional parameters" there as well see point 2 "adding entrypoints" here.

We are also missing a blurp for the backend itself, which we should add, I think, so that it is more like:

backend_name : backend_summary
    <additional docs if available>

Whether the additional parameters (if they exist) should be part of the additional docs or explicitly listed, I have no opinion for.
I am sure we can imagine a lot of additional tooling for the final deployed html docs!
Maybe we can manage to keep spatch simple, but just make things like additional_parameters availbe via a hook? I.e. a format_backend_docs() hook that is passed the information, and the default is just the above?

(I suppose that might get us eventually in the realm where subclassing BackendSystem to customize these functions makes sense.)

I suspect this isn't a huge priority or problem; mainly because it seems easy to extend. But as soon as a downstream project builds html docs and wants to include backends (or explicitly not include internal backends), we'll have to do something.