add CI action; rm stale files

.github/workflows/CompatHelper.yml

@@ -1,16 +0,0 @@
-name: CompatHelper
-on:
-  schedule:
-    - cron: 0 0 * * *
-  workflow_dispatch:
-jobs:
-  CompatHelper:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Pkg.add("CompatHelper")
-        run: julia -e 'using Pkg; Pkg.add("CompatHelper")'
-      - name: CompatHelper.main()
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          COMPATHELPER_PRIV: ${{ secrets.DOCUMENTER_KEY }}
-        run: julia -e 'using CompatHelper; CompatHelper.main()'