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

### Rationale for this change
Post  #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 <[email protected]>
Co-authored-by: Parth Chonkar <[email protected]>
Co-authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

README.md

@@ -45,7 +45,7 @@ Major components of the project include:
  - [Gandiva](https://github.com/apache/arrow/tree/main/cpp/src/gandiva):
    an [LLVM](https://llvm.org)-based Arrow expression compiler, part of the C++ codebase
  - [Go libraries](https://github.com/apache/arrow-go)
- - [Java libraries](https://github.com/apache/arrow/tree/main/java)
+ - [Java libraries](https://github.com/apache/arrow-java)
  - [JavaScript libraries](https://github.com/apache/arrow/tree/main/js)
  - [Python libraries](https://github.com/apache/arrow/tree/main/python)
  - [R libraries](https://github.com/apache/arrow/tree/main/r)

docs/source/developers/guide/step_by_step/arrow_codebase.rst

@@ -32,15 +32,18 @@ Working on the Arrow codebase 🧐
 Finding your way around Arrow
 =============================
 
-The Apache Arrow repository includes implementations for
-most of the libraries for which Arrow is available.
+The `Apache Arrow repository <https://github.com/apache/arrow>`_ includes
+implementations for most of the libraries for which Arrow is available.
 
 Languages like GLib (``c_glib/``), C++ (``cpp/``), C# (``csharp/``),
-Go (``go/``), Java (``java/``), JavaScript (``js/``), MATLAB
-(``matlab/``), Python (``python/``), R (``r/``) and Ruby (``ruby/``)
-have their own subdirectories in the main folder as written here.
+JavaScript (``js/``), MATLAB (``matlab/``), Python (``python/``), R (``r/``)
+and Ruby (``ruby/``) have their own subdirectories in the main folder as written here.
 
-Rust has its own repository available `here <https://github.com/apache/arrow-rs>`_.
+The following language implementations have their own repositories:
+
+- `Rust <https://github.com/apache/arrow-rs>`_
+- `Go <https://github.com/apache/arrow-go>`_
+- `Java <https://github.com/apache/arrow-java>`_
 
 In the **language-specific subdirectories** you can find the code
 connected to that language. For example: