Force maximum of Sphinx 3 for now

See #1851

.github/workflows/build.yml

@@ -111,7 +111,7 @@ jobs:
         python-version: 3
     - name: Install dependencies
       run: |
-        pip install sphinx
+        pip install 'sphinx<4'
     - name: Clone DFHack
       uses: actions/checkout@v1
       with:

docs/Documentation.rst

@@ -127,6 +127,10 @@ Required dependencies
 In order to build the documentation, you must have Python with Sphinx
 version |sphinx_min_version| or later. Python 3 is recommended.
 
+.. warning::
+
+  Sphinx 4 is currently not supported. See :issue:`1851` for details.
+
 When installing Sphinx from OS package managers, be aware that there is
 another program called Sphinx, completely unrelated to documentation management.
 Be sure you are installing the right Sphinx; it may be called ``python-sphinx``,