add Git topic, reorder topics alphabetically

doc/topics/index.md

@@ -7,8 +7,9 @@ you through better understanding GitLab's concepts
 through our regular docs, and, when available, through articles (guides,
 tutorials, technical overviews, blog posts) and videos.
 
-- [GitLab Installation](../install/README.md)
 - [Continuous Integration (GitLab CI)](../ci/README.md)
+- [Git](git/index.md)
+- [GitLab Installation](../install/README.md)
 - [GitLab Pages](../user/project/pages/index.md)
 
 >**Note:**