Switch to Antora Based Documentation

[rwinch]

Similar to Spring Security, Spring Session, and Spring Framework, Spring Boot should consider switching Antora Based Documentation.

A work in progress can be found at https://rwinch.github.io/spring-boot/boot/ and the source in https://github.com/rwinch/spring-boot/tree/antora

• Rebase the PR
• Switch to docsearch
• Verify extension use aligns with production Antora projects
  • Use root component extension https://github.com/spring-io/antora-extensions#root-component
  • Update versions
• Migrate Maven Plugin docs
• Migrate Gradle Plugin docs
• Migrate Actuator docs
• Verify build files align with production Antora projects
• Review
  • Do configprops work?
  • Does ID validation work?
  • Was it migrated to Asciidoctor tabs?
  • Have [null] references been fixed
  • Disable docs flag on zip