Update grails-doc with new path to integrate Micronaut

[jamesfredley]

New path: https://micronaut-projects.github.io/micronaut-spring/latest/guide/#springBootStarter

https://github.com/micronaut-projects/micronaut-spring/blob/5.8.x/spring-boot-starter/src/main/java/io/micronaut/spring/boot/starter/MicronautImportRegistrar.java

remove documentation for old integration which has been removed from Grails.

possibly point to examples in apache/grails-core#14017

[jamesfredley]

Changes in https://github.com/grails/grails-doc/pull/929/files#diff-8e980fe15c5064894b0dd674130fc48327c95705fd01b06afc3367aec8773b50 8.2