Add an XML schema for documentation

This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.

Each class XML's schema conformance is also checked on GitHub Actions.

.github/workflows/static_checks.yml

@@ -24,7 +24,7 @@ jobs:
 
       - name: Install dependencies
         run: |
-          sudo apt-get install -qq dos2unix recode clang-format-13
+          sudo apt-get install -qq dos2unix recode clang-format-13 libxml2-utils
           sudo update-alternatives --remove-all clang-format
           sudo update-alternatives --install /usr/bin/clang-format clang-format /usr/bin/clang-format-13 100
           sudo pip3 install black==21.10b0 pygments
@@ -48,6 +48,10 @@ jobs:
           npm run lint
           npm run docs -- -d dry-run
 
+      - name: Class reference schema checks
+        run: |
+          xmllint --noout --schema doc/class.xsd doc/classes/*.xml modules/*/doc_classes/*.xml
+
       - name: Documentation checks
         run: |
           doc/tools/make_rst.py --dry-run doc/classes modules