Use apt to provide sphinx and improve the workflow

.github/workflows/build.yml

@@ -1,31 +1,25 @@
 name: Build documentation
 
+permissions:
+    contents: read
+
 on:
-  push:
-  pull_request:
-    types: [opened, synchronize, reopened]
-    branches:
-      - master
+    push:
+    pull_request:
+        types: [opened, synchronize, reopened]
+        branches:
+            - master
 
 jobs:
     build-docs:
-        name: Build on python ${{ matrix.python-version }} and ${{ matrix.os }}
-        if: "!contains(github.event.head_commit.message, '[ci skip]')"
-        runs-on: ${{ matrix.os }}
-        strategy:
-            matrix:
-                python-version: ["3.x"]
-                os: [ubuntu-latest]
+        name: Build the documentation
+        runs-on: ubuntu-latest
         steps:
-            - uses: actions/checkout@v2
-              with:
-                submodules: true
-            - name: Use python ${{ matrix.python-version }}
-              uses: actions/setup-python@v2
+            - uses: actions/checkout@v3
               with:
-                python-version: ${{ matrix.python-version }}
+                  submodules: true
             - name: Install Sphinx
-              run: pip install Sphinx
+              run: sudo apt install sphinx-doc
             - name: Install gettext
               run: sudo apt-get install -y gettext
             - name: Build the documentation