add github actions

.github/workflows/main.yml

@@ -0,0 +1,18 @@
+name: build
+
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: xu-cheng/latex-action@v3
+      with:
+        root_file: lshort-zh-cn.tex
+        working_directory: src
+        args: -r latexmkrc
+    - uses: actions/upload-artifact@v4
+      with:
+        name: PDF
+        path: src/lshort-zh-cn.pdf