Add pending-extract attribute to source

Add `pending-extract=true` to source blocks to help us identify
those that need to be extracted to a real source file.

See gh-6313

spring-boot-project/spring-boot-docs/src/docs/asciidoc/getting-started.adoc

@@ -483,7 +483,7 @@ If you run `mvn dependency:tree` again, you see that there are now a number of a
 To finish our application, we need to create a single Java file.
 By default, Maven compiles sources from `src/main/java`, so you need to create that directory structure and then add a file named `src/main/java/Example.java` to contain the following code:
 
-[source,java,indent=0]
+[source,java,pending-extract=true,indent=0]
 ----
 	import org.springframework.boot.*;
 	import org.springframework.boot.autoconfigure.*;