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

bin/remove-git-folder

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+remove-git-folder() {
+    if [[ -z "$1" ]]; then echo "\$1 is empty."; exit; fi
+    local git_folders=$(ls-git-folder "$1")
+
+    echo "Delete: $git_folders"
+    read -p "Are you sure to delete these git folder(s)? (yes/N): " confirm
+    if [[ "${confirm,,}" != "yes" ]]; then
+        echo 'Aborted.'
+        return 1
+    fi
+    for git_folder in $git_folders; do
+        echo "Remove $git_folder from .gitmodules file."
+        git config --file .gitmodules --remove-section "submodule.$git_folder"
+
+        echo "Delete git folder $git_folder."
+        rm -rf "$git_folder"
+    done
+
+    save-version
+}