Speed up our Sphinx GitHub Action with ARM

# `runs-on: ubuntu-24.04-arm`
https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

Author: cclauss

.github/workflows/sphinx.yml

@@ -23,7 +23,7 @@ concurrency:
 
 jobs:
   build_docs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04-arm
     steps:
       - uses: actions/checkout@v4
       - uses: astral-sh/setup-uv@v5