Update docs workflow to use tox-uv

.github/workflows/docs.yaml

@@ -1,5 +1,12 @@
 name: Docs
 
+env:
+  # Current supported Python version. For applications, there is generally no
+  # reason to support multiple Python versions, so all actions are run with
+  # this version. Quote the version to avoid interpretation as a floating
+  # point number.
+  PYTHON_VERSION: "3.12"
+
 "on":
   push:
     branches:
@@ -23,16 +30,17 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: "3.12"
+          python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Install graphviz and ImageMagick
         run: sudo apt-get install graphviz imagemagick
 
       - name: Run tox
         uses: lsst-sqre/run-tox@v1
         with:
-          python-version: "3.12"
+          python-version: ${{ env.PYTHON_VERSION }}
           tox-envs: "docs"
+          tox-requirements: requirements/tox.txt
 
       - name: Upload documentation
         uses: lsst-sqre/ltd-upload@v1