Add info about Compiler Plugin to Migration Guide

Compiler Plugin was changed a lot, this all should be reflected in Migration Guide.
Also improve info about outdated plugins: https://kotlin.github.io/dataframe/migration-from-plugins.html

Start with the migration guide at https://kotlin.github.io/dataframe/migration-from-plugins.html and review how the Compiler Plugin and outdated plugins are currently described. Update the guide to reflect the substantial Compiler Plugin changes and improve the outdated-plugin information; done means both topics are accurately and clearly documented.

Written by the indexing model from the issue text.