Add documentation for Instance Statistics

This adds a documentation page to explain the Instance Statistics
feature, and a link to that page from the instance-level Analytics
documentation index.

doc/user/admin_area/analytics/index.md

@@ -4,7 +4,8 @@
 
 Administrators have access to instance-wide analytics, as shown in **Admin Area > Analytics**.
 
-There are two kinds of statistics:
+There are several kinds of statistics:
 
 - [DevOps Report](dev_ops_report.md): Provides an overview of your entire instance's feature usage.
+- [Instance Statistics](instance_statistics.md): Shows how much data your instance contains, and how that is changing.
 - [User Cohorts](user_cohorts.md): Display the monthly cohorts of new users and their activities over time.

doc/user/admin_area/analytics/instance_statistics.md

+# Instance Statistics
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/235754) in GitLab 13.4.
+
+Instance Statistics gives you an overview of how much data your instance contains, and how quickly this volume is changing over time.
+
+## Total counts
+
+At the top of the page, Instance Statistics shows total counts for:
+
+- Users
+- Projects
+- Groups
+- Issues
+- Merge Requests
+- Pipelines
+
+These figures can be useful for understanding how much data your instance contains in total.