Update group approval rules to settings

What does this MR do?

The current documentation for Group-level settings for merge request approvals refers the settings as "rules".

This may be misleading since the actual group MR approval rules have not been implemented yet - epic: Group-level settings for merge request approval RULES in child projects.

Users may be expecting to find group MR rules, similar to those for projects.

At the moment the project MR approval "rules" and "settings" are segregated accordingly in

(1) docs:

(2) and GitLab UI

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I have also added to the docs a link to the epic - not sure if the wording is alright?

Related issues

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