chore: update README and add GitHub Actions workflow for repository mirroring

- Updated the README to change the read-only mirror link to the new URL.
- Added a GitHub Actions workflow for mirroring the repository to a remote Git repository.

.github/workflows/mirror.yml

@@ -0,0 +1,17 @@
+name: Mirror
+on: [push]
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Mirror to remote repository
+        uses: yesolutions/mirror-action@master
+        with:
+          REMOTE: "https://git.popov.link/popov.link.git"
+          GIT_USERNAME: ${{ secrets.GIT_USERNAME }}
+          GIT_PASSWORD: ${{ secrets.GIT_PASSWORD }}