Docs: Add doc for shared cache

[qwint]

What is this fixing or adding?

adds doc file describing what the shared cache is, how to use it, and what you can currently expect in it

Note: includes the uuid cache in #5088 but since that is already approved for next release and this is just documentation I feel it's not actually blocked by that

How was this tested?

If this makes graphical changes, please attach screenshots.

[cjmang]

My only comment on this doc change is that to my knowledge, there is no Java library that obtains the relevant directories. I had thought that JNA might, but I'm not sure this is within its wheelhouse either.

So someone in Java (or some other language that doesn't have such a library) trying to follow this part of the documentation would have to dig into the exact implementation of a library that does follow spec, as the documentation here would be inadequate. And defacto AFAIK this is going to be platformdirs used by CommonClient.

[qwint]

some quick searching led to com.sun.deploy.config.Config.getCacheDirectory() does that not do what is necessary here?

[cjmang]

Using anything in the sun namespace is sketchy. Java's been removing APIs from there pretty regularly over the last 6 years.