Warning in doc for using Kotlin with older versions of Mandrel

[mihaipoenaru]

To avoid crashes like the one described in #25460, I've added a small warning label right at the start of the guide.

[gsmet]

Thanks, I amended your commit to improve the wording a bit.
Also we avoid referencing titles without a proper anchor as it's a bit brittle and can easily get broken when we adjust at title.

[mihaipoenaru]

Thanks, @gsmet ! I haven't worked with asciidoc before so I didn't know you could do that.

[gsmet]

Merged, thanks!