Add documentation

docs/en/user/admin/git.md

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+---
+title: Administration
+subtitle: Git
+---
+In the git section there are the following git specific settings:
+
+- GC Cron Expression
+
+    If this is set, SCM-Manager will execute a git garbage collection matching the given
+    [cron expression](https://en.wikipedia.org/wiki/Cron#CRON_expression).
+
+- Disable Non Fast-Forward
+
+    Activate this to reject forced pushes that are not fast forwards.
+
+- Default Branch
+
+    The branch name configured here will be used for the initialization of new repositories.
+    Please mind, that due to git internals this cannot work for empty repositories (here git
+    will always use its internal default branch, so at the time being `master`).
+
+![Administration-Plugins-Installed](assets/administration-settings-git.png)