CI: add action to sync master with main

This is the same action which we used to use in tldr.

Author: kbdharun

.github/workflows/mirror.yml

@@ -0,0 +1,18 @@
+name: Main Mirror
+
+on:
+  push:
+    branches: ['main']
+
+permissions: {}
+jobs:
+  mirror:
+    permissions:
+      contents: write # to update branch 
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: zofrex/mirror-branch@v1
+        with:
+          target-branch: master