"Edit on gitlab" button for documentation

In our documentation at secml.gitlab.io we are missing the "edit on gitlab" button, which is available as a standard in readthedocs (https://secml.readthedocs.io/en/stable/).

To generate this we need to simply add to docs/source/conf.py:

html_context = {
    "display_gitlab": True,
    "gitlab_host": "gitlab.com",
    "gitlab_user": "secml",
    "gitlab_repo": 'secml',
    "gitlab_version": "HEAD",
    "conf_py_path": "/docs/source/",
    "source_suffix": '.rst'
}