Improve documentation for CycloneDX integration

[wilkinsona]

For Gradle, how we react to the CycloneDX plugin being applied should be listed in https://docs.spring.io/spring-boot/gradle-plugin/reacting.html. For both Maven and Gradle, we'd like to make the build setup instructions currently located in https://docs.spring.io/spring-boot/reference/actuator/endpoints.html#actuator.endpoints.sbom a bit easier to find. The SBOM is somewhat analogous to the git.properties that's picked up by the info endpoint. Its build setup is documented at https://docs.spring.io/spring-boot/how-to/build.html#howto.build.generate-git-info so perhaps we need something similar for the SBOM stuff.