documentation for memory requirements when running H2 or Postgres

The repository manager system requirements for memory are written for when NXRM is using embedded OrientDB.

https://help.sonatype.com/repomanager3/installation/system-requirements#SystemRequirements-MemoryRequirements

It includes a formula for setting direct memory, namely because OrientDB consumes large quantities of it if allocated.

However now with H2 and Postgres, this changes in theory. Heap and direct memory allocations are expected to be different.

Expected

Document recommended memory requirements and formula when using either H2 or Postgres