apacheGH-611: [Docs] Detect version from pom.xml (apache#613)

Fixes apacheGH-611.

Author: kou

docs/source/conf.py

@@ -23,10 +23,17 @@
 # -- Project information -----------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#project-information
 
+import pathlib
+import xml.etree.ElementTree as ET
+
 project = 'arrow-java'
 copyright = '2025, Apache Arrow Developers'
 author = 'Apache Arrow Developers'
-release = '18.1.0'
+
+top_level_pom_xml = pathlib.Path(__file__).parents[2] / "pom.xml"
+tree = ET.parse(top_level_pom_xml)
+ns = {"maven": "http://maven.apache.org/POM/4.0.0"}
+release = tree.getroot().find("maven:version", ns).text
 
 # -- General configuration ---------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#general-configuration