TELESTION-446 Add GitHub action to publish

Add the GH Actions workflow file, already specifying the custom domain.

The workflow pushes the site to the `gh-pages` branch.

.github/workflows/docs.yml

@@ -0,0 +1,22 @@
+name: Deploy docs to GitHub Pages
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  deploy:
+    name: Deploy docs
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout main
+        uses: actions/checkout@v2
+      - name: Deploy docs
+        uses: mhausenblas/mkdocs-deploy-gh-pages@1.26
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          CUSTOM_DOMAIN: docs.telestion.wuespace.de
+          CONFIG_FILE: docs/mkdocs.yml
+          EXTRA_PACKAGES: build-base
+          REQUIREMENTS: docs/requirements.txt