CI for building doc with sphinx

Simplified from agda/agda/.github/workflows/user-manual.yml

.github/workflows/users-guide.yml

@@ -0,0 +1,61 @@
+# Adapted from agda/agda/.github/workflows/user-manual.yml by Andreas, 2021-09-11
+
+name: Users guide
+
+on:
+  push:
+    branches:
+    - master
+    paths:
+    - 'doc/requirements.txt'
+    - 'doc/*.inc'
+    - 'doc/*.py'
+    - 'doc/*.rst'
+    - 'doc/**/*.json'
+    - '.github/workflows/users-guide.yml'
+  pull_request:
+    paths:
+    - 'doc/requirements.txt'
+    - 'doc/*.inc'
+    - 'doc/*.py'
+    - 'doc/*.rst'
+    - 'doc/**/*.json'
+    - '.github/workflows/users-guide.yml'
+  release:
+    types:
+      - created
+
+jobs:
+  build:
+    if: |
+      !contains(github.event.head_commit.message, '[skip ci]')
+      && !contains(github.event.head_commit.message, '[ci skip]')
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.7]
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: recursive
+
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+
+    # Subsumed by make users-guide
+    # - name: Install dependencies
+    #   run: |
+    #     pip install -r doc/requirements.txt
+
+    - name: Build User's Guide in HTML
+      run: |
+        make SPHINX_HTML_OUTDIR=html users-guide
+
+    - uses: actions/upload-artifact@v2
+      with:
+        name: users-guide-html
+        path: html/