feat(git): add comprehensive git folder management commands and update documentation

bin/checkout-git-folder

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+checkout-git-folder() {
+    if [[ -z "$1" ]]; then
+        echo '$1/path is empty.'
+        exit
+    fi
+    if [[ -z "$2" ]]; then
+        echo '$2/path is empty.'
+        exit
+    fi
+
+    local git_branch="$1"
+    local git_folders=$(ls-git-folder "$2")
+    for git_folder in $git_folders; do
+
+        git_folder="${git_folder#./}"
+
+        local shallow=$(git config --file .gitmodules --get submodule."$git_folder".shallow)
+        local commit_ref=$(grep "$git_folder" ".gitmodules.csv" | cut -d',' -f2)
+
+        if [[ "$shallow" = "true" ]]; then
+            echo "Checkout git folder $git_folder (shallow) to $commit_ref"
+            git -C "$git_folder" stash
+            git -C "$git_folder" fetch origin "$git_branch:$git_branch"
+            git -C "$git_folder" checkout "$commit_ref"
+        else
+            echo "Checkout git folder $git_folder to $commit_ref"
+            git -C "$git_folder" stash
+            git -C "$git_folder" checkout "$commit_ref"
+        fi
+    done
+}