Drupal faq updates

https://docs.platform.sh/toolstacks/php/drupal/faq/

- [x] robots.txt are now moved by default, so the provided build hook is no longer valid
- [x] the page says Drupal 8 cannot be built from a make file, this is [out-of-date](https://github.com/platformsh/platformsh-examples/tree/drupal/8.x)
- [x] the page duplicates database import instructions which belong on [this page](https://docs.platform.sh/toolstacks/php/drupal/migrate-existing-site/#without-drush)
- [ ] we probably should still answer a "Can I run Drupal 8" question, linking to the platformsh-example