Remove obsolete docs folder

docs/README.adoc

-= NOTE
-
-Documentation can now be found in the `spring-boot-docs` project. This folder will be
-removed at a future date.

docs/howto.md

-# How Do I Do That With Spring Boot?
-
-**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`](../spring-boot-docs/src/main/asciidoc/howto.adoc).