GH-47974: [Docs] Remove stray documentation from Java and JS (#48006)

### Rationale for this change

The documentation was migrated to their respective repositories 

### What changes are included in this PR?

Remove Java and JS stray documentation.
Point Java docs link to https://arrow.apache.org/java/

### Are these changes tested?

No

### Are there any user-facing changes?

No

* GitHub Issue: #47974

Authored-by: Raúl Cumplido <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: raulcd

docs/source/implementations.rst

@@ -47,7 +47,7 @@ documentation and source code for these libraries.
      - `Go Docs <https://arrow.apache.org/go/>`_ :fa:`external-link-alt`
      - `Go Source <https://github.com/apache/arrow-go>`_
    * - Java
-     - :doc:`Java Docs<java/index>`
+     - `Java Docs <https://arrow.apache.org/java/>`_ :fa:`external-link-alt`
      - `Java Source <https://github.com/apache/arrow-java>`_
    * - JavaScript
      - `JavaScript Docs <https://arrow.apache.org/js/>`_ :fa:`external-link-alt`