reactivate some old workflows, remove some old workflows (#80)

I deleted most workflows from apps/backend/.github/workflows because imo
we don't need them in the near future. I updated and moved two workflows
which we do need. Only the codeQL workflow is left behind, because I
don't have an opinion yet whether it should be removed or kept.

.github/workflows/generate-theme-doc.yml

@@ -0,0 +1,53 @@
+name: Generate Theme Readme
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - "apps/backend/themes/index.js"
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  generate-theme-doc:
+    runs-on: ubuntu-latest
+    name: Generate theme doc
+
+    permissions:
+      actions: read
+      checks: read
+      contents: write
+      deployments: read
+      issues: read
+      discussions: read
+      packages: read
+      pages: read
+      pull-requests: write
+      repository-projects: read
+      security-events: read
+      statuses: read
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+
+      - name: Install Dependencies
+        uses: ./.github/actions/install-dependencies
+
+      - name: Generate readme
+        run: |
+          pnpm --filter github-readme-stats run theme-readme-gen
+
+      - name: Create Pull Request if themes README has changed
+        uses: peter-evans/create-pull-request@8
+        with:
+          commit-message: "feat(backend): update themes README"
+          branch: "update_themes_readme/patch"
+          delete-branch: true
+          title: Update themes README
+          body: "The
+            [generate-theme-doc](https://github.com/stats-organization/github-stats-extended/actions/workflows/generate-theme-doc.yml)
+            action found new/updated languages in the [upstream languages JSON
+            file](https://raw.githubusercontent.com/github/linguist/master/lib/linguist/languages.yml)."
+          labels: "ci"