Link to user-facing docs for background migrations

What does this MR do and why?

This changes the "Learn more" link in the Background Migrations admin area to go to our user-facing documentation (https://docs.gitlab.com/ee/user/admin_area/monitoring/background_migrations.html) instead of the developer-targeted documentation it is linking to currently (https://docs.gitlab.com/ee/development/database/batched_background_migrations.html). As this is UI that is meant for use by our end-users, this link should surface more relevant information for most of them.

Screenshots or screen recordings

No visible changes, as only a link target is modified.

How to set up and validate locally

I did not test this locally, as I don't have GDK set up – I compared to other help_page_path() usage in the codebase to make sure I change it the right way. I'd appreciate someone experienced to 100% verify this, though.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.