Polish "Support provided-scope libraries when extracting an executable war"

Make the extracted war runnable with java -jar: move root content
such as static files and JSPs to META-INF/resources/, name the
application jar *.jar, and write the META-INF/ directory entry
before the manifest.

Fail early when a library and a provided library share a file
name, as both would be extracted to the same location. Assert the
Spring-Boot-Lib-Provided attribute in the Maven and Gradle tests
and document war extraction.

See gh-51372

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/packaging/efficient.adoc

@@ -33,6 +33,10 @@ $ java -jar my-app/my-app.jar
 
 After startup, you should not expect any differences in execution time between running an executable jar and running an extracted jar.
 
+NOTE: An executable war can be extracted in the same way.
+The result is a jar, for example `my-app/my-app.jar`, that is meant to be run with `java -jar`.
+Content from the root of the war, such as static files and JSPs from `src/main/webapp`, is moved to `META-INF/resources/` in the extracted jar.
+
 TIP: Run `java -Djarmode=tools -jar my-app.jar help extract` to see all possible options.