ci: add vimdoc workflow

Tidy doesn't have any vim documentation. This
commit adds a workflow that automatically creates
a vimdoc from our readme, then commits it back to
the repo.

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: CI
+on:
+  push:
+    paths:
+      - "README.md"
+  pull_request:
+    paths:
+      - "README.md"
+
+jobs:
+  docs:
+    runs-on: ubuntu-latest
+    if: ${{ github.ref == 'refs/heads/main' }}
+    name: pandoc markdown to vimdoc
+    steps:
+      - uses: actions/checkout@v3
+      - name: panvimdoc
+        uses: kdheepak/panvimdoc@main
+        with:
+          vimdoc: tidy.nvim
+          version: "Neovim >= 0.9.0"
+          demojify: true
+          treesitter: true
+      - name: Push changes
+        uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: "chore(build): auto-generate vimdoc"