arrow

MINOR: [Docs] Update docs to point to separate Java codebase (#45134)



### Rationale for this change
Post  https://github.com/apache/arrow/issues/44945 the Java implementation lives in it's own repo. Update docs
to point there. 

### What changes are included in this PR?
Updates to a few locations that reference old Java impl location.

### Are these changes tested?
Rendered the Sphinx ones locally to check.  

### Are there any user-facing changes?
No

Lead-authored-by: parthchonkar <parthchonkar@gmail.com>
Co-authored-by: Parth Chonkar <parthchonkar@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>