Reinstate single page reference documentation

This commit reinstates the single-page variant for the Spring Boot
reference documentation.

Now the single and multi-page variants are hosted under the same
`"html"` folder, as `"index.html"` and `"single-index.html"`.
Note that files ending in `"index.html`" are treated as index files by
the spring-doc-resources project and their table of contents do not
display a "Back to index" link.

Fixes gh-16295

spring-boot-project/spring-boot-docs/src/main/asciidoc/documentation-overview.adoc

@@ -12,6 +12,9 @@ as a map for the rest of the document.
 
 [[boot-documentation-about]]
 == About the Documentation
+The Spring Boot reference guide is available as
+<<single-index.adoc#spring-boot-reference-documentation, single page>> or
+<<index.adoc#spring-boot-reference-documentation, multi-page>> documentation.
 The latest copy
 of the reference documentation is available at {spring-boot-docs-current}.
 

spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc

@@ -1,3 +1,4 @@
+[[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

spring-boot-project/spring-boot-docs/src/main/asciidoc/single-index.adoc

@@ -0,0 +1,17 @@
+[[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]
+include::appendix.adoc[leveloffset=+1]