Add ehrQL Pygments configuration

We now use `ehrql` syntax labels for complete dataset definitions in the
ehrQL documentation: opensafely-core/ehrql#1648.

This documentation repository needs to know about this configuration
too. This duplication is a little bit irritating, but it is because our
multi-repository configuration doesn't have any kind of configuration
inheritance or reuse. See opensafely-core/ehrql#1126

A future improvement might be to use the Material for MkDocs projects
plugin once it's available in the free version which might also fix this
duplication: #1317.

mkdocs.yml

@@ -167,7 +167,10 @@ markdown_extensions:
           format: !!python/name:pymdownx.superfences.fence_code_format
   - toc:
       permalink: "🔗"
-  - pymdownx.highlight
+  - pymdownx.highlight:
+      extend_pygments_lang:
+        - name: ehrql
+          lang: python
   - pymdownx.superfences
   - pymdownx.tabbed:
       alternate_style: true