Move appendix subsections under appendix section

This involved a small code change to the generated configuration
properties snippets. The section id has to start with
'appendix.', otherwise the section-id asciidoctor extension
complains. To ensure that the anchors that are derived from the
section IDs remain backwards compatible, the anchor-rewrite
properties have been updated.

See gh-29667

spring-boot-project/spring-boot-docs/src/docs/asciidoc/executable-jar.adoc

@@ -1,5 +1,5 @@
 [appendix]
-[[executable-jar]]
+[[appendix.executable-jar]]
 = The Executable Jar Format
 include::attributes.adoc[]