doc: encourage using group access token instead of personal access token

Don't panic, we provide all required tools to initiate it for the first time, and also schedule regular updates to keep synchronized.

1. Create an empty `to-be-continuous` root group with `public` visibility.

2. Create a dedicated _non-individual_ GitLab account, add it to the `to-be-continuous` root group

   with `Maintainer` role.

3. Generate a [personal access token](https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html) with scopes `api,read_registry,write_registry,read_repository,write_repository`.

2. In the `to-be-continuous` root group, generate an [access token](https://docs.gitlab.com/ee/user/group/settings/group_access_tokens.html) with scopes `api,read_registry,write_registry,read_repository,write_repository` and with `Owner` role.

3. Store the token in `GITLAB_TOKEN` environment variable.

4. Execute the following command to **recursively copy the tbc group**:

    ```bash

    curl -s https://gitlab.com/to-be-continuous/tools/gitlab-sync/-/raw/master/gitlab-sync.sh | bash /dev/stdin --dest-api https://your.gitlab.host/api/v4 --dest-token $GITLAB_TOKEN --exclude samples,custom