Remove references to personal preferences toggle for code suggestions in Docs

In conjunction with https://gitlab.com/gitlab-org/gitlab/-/issues/435269, we should remove the instructions to enable the personal preferences toggle in documentation and update them such they still make sense. This should include references in the main code suggestions doc pages as well as references in the extensions.

We should also make clear that personal preferences toggle was never necessary for SM users.

Lastly, because we will have a rolling rollout of the removal, some users will need to enable the toggle, others will not. We need to find a solution to elegantly make that clear to both types of users, at least temporarily.