(Part 9) Multiple approval rules - docs

What does this MR do?

This MR updates the merge_request_approval docs for the new "Multiple approval rules" feature. Please note that it points to a feature branch (not master) and that the changes described in the docs are behind a feature flag (disabled for now).

Related issues

Part of #1979 (closed)

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

/label ~Documentation