doc: add basic Gradle setup using Kotlin

[SimonVerhoeven]

What's changed?

I added a basic setup for those using Gradle + Kotlin

What's your motivation?

Making the basic setup easier for the readers, especially as Kotlin has become the default for gradle

Anything in particular you'd like reviewers to focus on?

no

Anyone you would like to review specifically?

Have you considered any alternatives or workarounds?

no

Any additional context

Checklist

• I've added unit tests to cover both positive and negative cases
• I've added the license header to any new files through ./gradlew licenseFormat
• I've used the IntelliJ auto-formatter on affected files
• I've updated the documentation (if applicable)

[timtebeek]

Thanks a lot for adding this! It's indeed been missing in the docs, but should become more prevalent.

@mike-solomon would you mind going over this? Ties in nicely with the plans we had over on openrewrite/rewrite-recipe-markdown-generator#66

[mike-solomon]

Looks good to me. One minor capitalization fix -- but otherwise looks great. Appreciate you taking the time to add this!