Document how to enable/disable Auto DevOps at the instance level

Achilleas Pipinellisrequested to merge
docs/autodevops-admin into master

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49820

I also added a very draft index file for all currently documented settings.

Edited by 🤖 GitLab Bot 🤖

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