CI for all three forges: build README.html and publish as artifact

.forgejo/workflows/build-readme.yml

@@ -0,0 +1,30 @@
+name: Build README.html
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: pandoc/core:3.1
+
+    steps:
+      - name: Checkout
+        # Forgejo documents that actions/checkout can be referenced via DEFAULT_ACTIONS_URL,
+        # or explicitly via a Forgejo-hosted URL.
+        uses: actions/checkout@v4
+        # Alternatively:
+        # uses: https://data.forgejo.org/actions/checkout@v4
+
+      - name: Build README.html
+        run: ./scripts/build_readme.sh
+
+      - name: Upload artifact
+        # Forgejo docs recommend v3 (or patched v4).
+        uses: https://code.forgejo.org/actions/upload-artifact@v3
+        with:
+          name: README-html
+          path: README.html

.gitea/workflows/build-readme.yml

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+name: Build README.html
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: pandoc/core:3.1
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build README.html
+        run: ./scripts/build_readme.sh
+
+      # If Gitea instance can fetch GitHub actions:
+      - name: Upload artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: README-html
+          path: README.html

.gitlab-ci.yml

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+stages: [build]
+
+build_readme_html:
+  stage: build
+  image: pandoc/core:3.1
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "push"
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+  script:
+    - ./scripts/build_readme.sh
+  artifacts:
+    name: "readme-html-$CI_COMMIT_SHORT_SHA"
+    paths:
+      - README.html
+    expire_in: 30 days

scripts/build_readme.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+in="README.md"
+out="README.html"
+
+if [[ ! -f "$in" ]]; then
+  echo "ERROR: $in not found" >&2
+  exit 1
+fi
+
+# GitHub-flavored Markdown -> standalone HTML
+pandoc "$in" \
+  --from=gfm \
+  --to=html5 \
+  --standalone \
+  --metadata title="README" \
+  --output "$out"
+
+echo "Built $out"