Split the Ubuntu jobs into a PR set, a full set, and a nightly set

All ~100 Ubuntu jobs ran on every push to every pull request. They now
belong to three sets, chosen by each job's if: condition:

- PR set (16 configurations): every push to a pull request and every
  push. Together with the checks that always run, these jobs would
  have caught at least 92 % of the failing commits of Sep/Oct 2026.
- full set (73 configurations): after the PR set passed, on pushes and
  on pull requests with the "🚀 ready to merge" label.
- nightly set (10 configurations): Infer, Valgrind, coverage, icpc,
  icpx, nvhpc, CUDA, and ARM64; nightly on develop, on pushes to master
  and release/*, and by hand. A failed nightly run opens or updates an
  issue.

A matrix whose entries belong to different sets is split into one job
per set; the jobs share their steps through YAML anchors. No
configuration is removed: tools/ci_jobs/ci_jobs.py --compare lists 150
configurations before and after. The same tool checks in CI that every
job belongs to exactly one set, that the full set waits for the whole
PR set, and that the job table in quality_assurance.md is up to date.

The documentation describes the sets in a new section "How CI runs",
and the QA pipeline figure shows the new badges.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/repository_checks.yml

@@ -2,7 +2,7 @@ name: Repository checks
 
 # Checks that concern files the compiler workflows (ubuntu.yml, macos.yml,
 # windows.yml) skip with their path filter: the documentation, the license
-# information, and the path filter itself. This workflow has no path filter.
+# information, and the CI configuration itself. This workflow has no path filter.
 on:
   push:
     branches:
@@ -20,7 +20,7 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  check_path_filters:
+  check_ci_configuration:
     runs-on: ubuntu-26.04
     steps:
       - name: Harden Runner
@@ -33,6 +33,11 @@ jobs:
           persist-credentials: false
       - name: Check the path filters of the compiler workflows
         run: python3 tools/check_path_filters/check_path_filters.py
+      - name: Check the job sets of the compiler workflows and their documentation
+        run: |
+          python3 -m venv venv
+          venv/bin/pip install --quiet -r tools/ci_jobs/requirements.txt
+          venv/bin/python tools/ci_jobs/ci_jobs.py
 
   ci_reuse_compliance:
     runs-on: ubuntu-26.04