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/deployment.adoc

@@ -135,7 +135,7 @@ Process-scoped environment variables are language agnostic.
 
 Environment variables do not always make for the easiest API, so Spring Boot automatically extracts them and flattens the data into properties that can be accessed through Spring's `Environment` abstraction, as shown in the following example:
 
-[source,java,indent=0]
+[source,java,pending-extract=true,indent=0]
 ----
 	@Component
 	class MyBean implements EnvironmentAware {