Merge pull request juju#18427 from Aflynn50/make-docs-job-periodic

juju#18427

This job was running on every push to every pull request. Make it periodic as its quite expensive.

Also move to the self hosted runners.
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<!-- Why this change is needed and what it does. -->

## QA steps

Check workflow when it runs. (Workflow has been passing on every PR before so is in good state.)
<!-- Describe steps to verify that the change works. -->

Author: jujubot

.github/workflows/docs-sphinx-python-dependency-build-checks.yml

@@ -11,9 +11,10 @@
 # succeed.
 name: Check and document build requirements for Sphinx venv
 on:
-  - push
-  - pull_request
-  - workflow_dispatch
+  schedule:
+    - cron: "0 2 * * 1,5" # Runs at 02:00 AM on every Monday and Friday.
+  workflow_dispatch:
+
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -22,7 +23,7 @@ concurrency:
 jobs:
   build:
     name: build
-    runs-on: ubuntu-latest
+    runs-on: [linux, self-hosted, x64, large, jammy]
     steps:
       - name: Checkout code
         uses: actions/checkout@v4