[Java] Bootstrap initial developer documentation in docs/source/developers/java.rst

[asfimport]

The project lacks prose documentation about Java development. I propose to begin a section about it in the Sphinx project

Reporter: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-5542. Please see the migration documentation for further details.}

[lidavidm]

We have development documentation already.