Remove duplication in documentation index files

This commit combines index adoc files for single-page HTML and PDF
into one file to remove duplication. It also renames index files to
reflect the separation between source format and generated output
format.

Finishes gh-20829

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-reference.pdfadoc

@@ -1,28 +0,0 @@
-[[spring-boot-reference-documentation]]
-= Spring Boot Reference Documentation
-Phillip Webb, Dave Syer, Josh Long, Stéphane Nicoll, Rob Winch, Andy Wilkinson, Marcel Overdijk, Christian Dupuis, Sébastien Deleuze, Michael Simons, Vedran Pavić, Jay Bryant, Madhura Bhave
-:docinfo: shared
-include::attributes.adoc[]
-
-include::legal.adoc[leveloffset=+1]
-include::documentation-overview.adoc[leveloffset=+1]
-include::getting-started.adoc[leveloffset=+1]
-include::using-spring-boot.adoc[leveloffset=+1]
-include::spring-boot-features.adoc[leveloffset=+1]
-include::production-ready-features.adoc[leveloffset=+1]
-include::deployment.adoc[leveloffset=+1]
-include::spring-boot-cli.adoc[leveloffset=+1]
-include::build-tool-plugins.adoc[leveloffset=+1]
-include::howto.adoc[leveloffset=+1]
-
-
-
-[[appendix]]
-== Appendices
-
-include::appendix-application-properties.adoc[leveloffset=+2]
-include::appendix-configuration-metadata.adoc[leveloffset=+2]
-include::appendix-auto-configuration-classes.adoc[leveloffset=+2]
-include::appendix-test-auto-configuration.adoc[leveloffset=+2]
-include::appendix-executable-jar-format.adoc[leveloffset=+2]
-include::appendix-dependency-versions.adoc[leveloffset=+2]