Merge branch 'docs/ssot_admin_area_index' into 'master'

Edit admin index to comply with SSOT guidelines

See merge request gitlab-org/gitlab-ce!28569

doc/user/admin_area/index.md

+---
+type: reference
+---
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 # GitLab Admin Area **[CORE ONLY]**
 
 The Admin Area provides a web UI for administering some features of GitLab self-managed instances.