Add doc about the TransientStore

Add a global documentation

When refactored and API finally finalized, see NXP-18050 and NXP-18051.
Describe purpose, use and main implementations.

Update Nuxeo Redis doc with a section about the RedisTransientStore.

https://doc.nuxeo.com/display/NXDOC/Nuxeo+and+Redis

Explain how to configure the TransientStore:

• Override default store config
• Configure the TransientStore used for:
  • Batch upload: BatchManagerCache
  • TransientStoreWork (Conversions): transientStoreWorkCache
  • Renditions: LazyRenditionCache

Check existing documentation about sharing the caching directory

https://doc.nuxeo.com/display/ADMINDOC/Nuxeo+Clustering+Configuration#NuxeoClusteringConfiguration-TransientStore.