Add GH Action to build and publish documentation

.github/workflows/docs.yml

+name: Generate and publish Android Matrix SDK documentation
+
+on:
+  push:
+    branches: [ main ]
+
+jobs:
+  docs:
+    name: Docs
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Build docs
+        run: ./gradlew dokkaHtml
+
+      - name: Deploy docs
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./matrix-sdk-android/build/dokka/html