Add documentation about the `memory` storage

[n-peugnet]

It seems there is a memory storage type that exists, but it is not documented. One can however find it in multiple places:

1. in the docs welcome page (see the memory icon):
   

2. in helm charts examples:

   Dex requires a minimal configuration in order to work. You can pass configuration to Dex using Helm values:

   config:
     # Set it to a valid URL
     issuer: http://my-issuer-url.com
   
     # See https://dexidp.io/docs/storage/ for more options
     storage:
       type: memory
   
     # Enable at least one connector
     # See https://dexidp.io/docs/connectors/ for more options
     enablePasswordDB: true

3. in the sources: https://github.com/dexidp/dex/blob/master/storage/memory/memory.go

   // Package memory provides an in memory implementation of the storage interface.
   package memory
   [...]

But there are no more information about it. I wonder what it is good for, in what situation it is interesting to use it, or if it can really be used at all?

[n-peugnet]

Can I have at least a quick answer about it in this thread?