[CI] Add workflow for witx doc.

.github/workflows/doc.yml

+name: doc
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  gen_doc:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: rustup
+      run: |
+        rustup update stable --no-self-update
+        rustup default stable
+
+    - name: generate md from witx       # XXX: md files goes nowhere
+      run: ./generate.sh
+      working-directory: utils/witx-doc