The How-to is now part of the reference documentation.
The How-to guide has moved and is now published as part of the reference documentation. You can find the latest browsable copy at http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#howto
If you want to contribute to the How-to the source is available at
spring-boot-docs/src/main/asciidoc/howto.adoc.