TPASPKT-1017 Publish Sphinx docs using GitHub Pages

Publish Sphinx docs to GitHub Pages using GitHub Actions CI service.

The actual docs are located at `<organization>.github.io/<repo>`.

.github/workflows/documentation.yml

@@ -0,0 +1,46 @@
+---
+name: documentation
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+jobs:
+  build:
+    # Ubuntu 20.04 has Python 3.9
+    runs-on: ubuntu-20.04
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: "3.9"
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -r requirements_github.txt
+          python -m pip install sphinx sphinx-autodoc-typehints
+      - name: Run Sphinx build
+        run: make doc
+      - name: Upload doc artifacts
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: ./doc/build/html
+
+  deploy:
+    # Only deploy on 'master' branch. It's probably still useful to check
+    # if the docs actually build on other branches, too.
+    if: github.ref == 'refs/heads/master'
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

README.rst

@@ -22,7 +22,9 @@ After the repository has been added, the package can be installed by running the
 
 Usage
 -----
-See documentation under ``doc/``.
+See the `online documentation`_.
+
+.. _online documentation: https://digital-preservation-finland.github.io/dpres-mets-builder
 
 Installation using Python Virtualenv for development purposes
 -------------------------------------------------------------