add documentation about git

docs/user-guide/git.md

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+# Git
+
+Git is a distributed version control system used primarily for tracking changes in source code and other files over time. It is currently regarded as the standard for this type of purpose.
+
+There are various resources to learn about Git:
+
+- Git course: https://rse.psi.ch/events/2026-04-28/Git-course/
+- https://ohmygit.org
+- ...
+
+The central Git server for PSI is: https://gitea.psi.ch