feat(docs): Add note about GitLab Client-Side Implicit Grant
feat(docs): Add note about GitLab Client-Side Implicit Grant

## Description

As per comment https://github.com/netlify/netlify-cms/issues/1690#issuecomment-465078677
Identity widget conflicts with implicit authentication and should be disabled.




Co-authored-by: Lennart <074ba1c0efc36927866875b17be4fa06eb80e472@gmail.com>
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