Add kotlin and kte documentation

[checketts]

I would like to add to the documentation a section specific to getting up and running with Kotlin and KTE templates.

Points I've run into that would be good to document:

1- Kotlin templates need to be named with .kte, you can't use Kotlin code in a .jte file.
2- Add the jte-kotlin dependency using compileOnly so it doesn't bring in the Kotlin compiled (~60mb)
3- Various gotchas and error messages

• Non-null parameters missing from a template (error isn't clear, but recognizable once it is known)
• Null lists in a @for block error messages
  4- Sub-tempate syntax:

!{val subTemplate = {name -> @`<div>Hello ${name}!</div>`}}
${subTemplate("World")}
${subTemplate("JTE")}

Would these additions be welcome as a new section in the docs?

[casid]

Welcome @checketts!

Thanks for sharing your experience, that sounds great.

Very happy to receive a PR from you for these improvements!