Add `enable` option to the OmniAuth documentation

What does this MR do and why?

Add enable option to the OmniAuth documentation

Users were trying to disable the OmniAuth service by using allow_single_sign_on instead of the enable options. Both serve different purposes.

Here, we add documentation for the enable option which disable the whole OmniAuth provider service and hides all the OmniAuth provider buttons.

See first point raised in #415478 (closed)

MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

Documentation page affected: https://docs.gitlab.com/ee/integration/omniauth.html#configure-common-settings