Merge duplicate Boot plugin sections and add layertools section

[scottfrederick]

This PR updates the content of this guide with the following changes:

• Merges duplicate Spring Boot Plugins and Spring Boot Maven and Gradle Plugin sections
• Updates sample output from Spring Boot plugins to match current actual output
• Adds a section on Spring Boot layer tools as an alternative to manually unzipping jar contents into layers
• Updates Buildpacks section to reflect Paketo builder and buildpacks as the current CNB implementation

The demo application has also been updated to use the Spring Boot layertools technique for layering.

[scottfrederick]

@dsyer Would you mind taking a look at these changes when you get a chance (github won't let me suggest you as a reviewer)?