New MV Implementation - Documentation & Migration Guide


The adapter's documentation should be updated with the new MV model definitions and migration options.

### Description

Add documentation on:

* How to define `target_table` and `materialized_view` models.

* Example configuration using `ref()`.

* Backfill strategies.

* Migration guide for existing projects.

* Make sure we update the docs both on ClickHouse.com and the dbt site. 