Document "non-React mode"

[zth]

With the new APIs in Relay 18.2 (waitForFragmentData) it's now entirely possible to run Relay without React. A few more APIs need bindings for this to be really slick, but the query->fragment composition model already works very well with something like res-x.

Document that there's now a mode in RescriptRelay where React-specific hooks etc can be configured to not be emitted.