docs: Add comment for use of Ubuntu 20 for a11y.yml

.github/workflows/a11y.yml

@@ -3,6 +3,7 @@ on: [push, pull_request]
 jobs:
   build:
     if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+    # https://github.com/pa11y/pa11y-ci#pa11y-ci-3-and-ubuntu
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v4