Extract spring-doc-resources URL property

Update the build to use a shared spring-doc-resources URL
rather than repeating it multiple times.

See gh-18293

spring-boot-project/spring-boot-docs/pom.xml

@@ -1193,7 +1193,7 @@
 									<goal>wget</goal>
 								</goals>
 								<configuration>
-									<url>https://repo.spring.io/snapshot/io/spring/docresources/spring-doc-resources/${spring-doc-resources.version}/spring-doc-resources-${spring-doc-resources.version}.zip</url>
+									<url>${spring-doc-resources.url}</url>
 									<unpack>true</unpack>
 									<outputDirectory>${refdocs.build.directory}</outputDirectory>
 								</configuration>