Add doc review to SPI changes in https://github.com/infinispan/infinispan/pull/8370

this changes how a user would add a custom store - it is the same pluggability as before, but I hope it is a bit simpler to understand, albeit possibly a bit harder to implement as they need to be non blocking when using this SPI

existing store implementations are unaffected currently as I added an adapter that does the conversion for them. However the goal is to eventually remove the old SPIs, probably not until ISPN 14 though according to our deprecation/removal schedule