Add gitlab robot user workaround to documentation

[Refoo0]

GitLab provides group and project access tokens as a paid feature. We are already in the process of creating an autosetup functionality for gitlab to simplify the setup process, since group webhooks cant be used as well.

To workaround the permission limitation, eg. I am part of two repositories but only want devguard to access a single one, is by creating a robot user: Just create a new account, generate a personal access token and add the user to the repositories one would like to share. Effectivly circumventing the paid feature and providing similar functionality. We should help our users by documenting how todo that.