git-submodule: Add tldr page for the git-submodule command.

Update git-submodule to pass linter requirements.

pages/common/git-submodule.md

@@ -0,0 +1,15 @@
+# git submodule
+
+> Inspects, updates and manages submodules.
+
+- Install a repository's specified submodules:
+
+`git submodule update --init --recursive`
+
+- Add a git repository as a submodule:
+
+`git submodule add {{repository-url}}`
+
+- Update every submodule to its latest commit:
+
+`git submodule foreach git pull`