Add values for "merge_method" to documentation

The documentation for attribute merge_method in resource gitlab_project is pretty sparse. It should not only mention ff

(String) Set to ff to create fast-forward merges

but all values that are possible.

Per https://gitlab.com/gitlab-org/terraform-provider-gitlab/-/blob/main/internal/provider/resource_gitlab_project.go#L150 those are

• merge
• rebase_merge
• ff

See also https://docs.gitlab.com/ee/api/projects.html#project-merge-method