[examples] Migrate examples to composite builds (JetBrains#810)

I've updated this PR to actually migrate `examples` to a separate
`simple-examples` Gradle project in `examples` directory, that depends
on the root Koog project via composite build. `examples` is not a module
in Koog anymore, just a directory with example projects.

After some experiments, I decided not to migrate other example projects,
such as Demo Compose App, to composite build approach, at least for now.
I believe it's more beneficial to keep these logically uncoupled from
the main project, with exact pinned version. Makes it easier to work on,
and also doesn't make CI run Demo App checks on every PR.

Also, updated READMEs to reflect these changes.

Author: EugeneTheDev

settings.gradle.kts

@@ -90,3 +90,10 @@ include(":docs")
 
 include(":test-utils")
 include(":utils")
+
+/*
+ Some separate Gradle projects with examples.
+ Included for convenience to be able to run them from the root project.
+ Main Koog project (this one) build does not depend on them in any way.
+ */
+includeBuild("./examples/simple-examples")