ci: Add docs assignment-table auto-updater

Regenerate the marked network-assignment tables in docs/ from
nixos.allAssignments (nix run .#update-docs-assignments, registered in
pkgs/default.nix), preserving hand-written Notes cells. The workflow
runs on pushes to the docs branch and commits any table updates back;
flip its trigger to master when the docs branch merges.

.gitea/workflows/update-docs.yaml

@@ -0,0 +1,38 @@
+name: Update docs assignments
+
+on:
+  push:
+    branches: [docs-kimi-new]
+
+jobs:
+  update:
+    if: "!contains(github.event.head_commit.message, 'docs: update assignment tables')"
+    runs-on: ubuntu-26.04
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: cachix/install-nix-action@v31
+        with:
+          github_access_token: ${{ secrets.GH_PULL_TOKEN }}
+          extra_nix_config: |
+            extra-substituters = https://nix-cache.nul.ie
+            extra-trusted-public-keys = nix-cache.nul.ie-1:BzH5yMfF4HbzY1C977XzOxoPhEc9Zbu39ftPkUbH+m4=
+
+      - name: Update assignment tables
+        run: nix run .#update-docs-assignments
+
+      - name: Commit and push if changed
+        env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+          REPO_URL: ${{ gitea.repositoryUrl }}
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git remote set-url origin "${REPO_URL/https:\/\//https:\/\/oauth2:${GITEA_TOKEN}@}"
+          git add docs/
+          if ! git diff --cached --quiet; then
+            git commit -m "docs: update assignment tables"
+            git push
+          fi