Create redirect file, and stubs for new pages

I'll need to split this page apart into three separate pages (four,
counting the index) so let's start by adding in the redirect file
for the old page, and the stubs for the pages I'm about to carve out.

doc/user/project/repository/repository_mirroring.md

+---
+redirect_to: 'mirror/index.md'
+remove_date: '2022-03-22'
+---
+
+This document was moved to [another location](mirror/index.md).
+
+<!-- This redirect file can be deleted after <2022-03-22>. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->
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