Mention flask-sqlalchemy-lite as a supported engine

[ElLorans]

Flask-Admin does not explicitly support Flask-Sqlalchemy-Lite, but since it basically acts the same as a Sqlalchemy model and we use only the Sqlalchemy features, there is no difference.
We should at a minimum mention that it is possible to use it, and at best add testing and type hints.

[samuelhwilliams]

It would be good to officially support; if we do call it out in docs we should definitely add test coverage for it.