Add a FAQ entry about migrating tests to use JUnit Jupiter API.

Describe best-practices of migrating `@run main` annotated tests.

For tests using external frameworks link to FAQs hosted at JUnit website. Like:
- https://github.com/junit-team/junit5/wiki/Migrating-from-JUnit-4-to-JUnit-Jupiter
- https://github.com/junit-team/junit5/wiki/Migrating-from-TestNG-to-JUnit-Jupiter