Add the wiki publishing workflow

Add tools/publish_wiki.py and the Publish wiki workflow, which make the
Gitea wiki equal wiki/ through the wiki REST API when a change to wiki/ is
merged to main. It needs its own secret TOKEN_FOR_GITEA_WIKI, refuses an
empty source and checks that the English pages are current first.

Document the secret in the credentials and troubleshooting guides and
regenerate the affected wiki pages.

Task: MIL-005#8

.gitea/workflows/publish-wiki.yml

@@ -0,0 +1,31 @@
+name: Publish wiki
+
+on:
+  push:
+    branches: [ main ]
+    paths:
+      - "wiki/**"
+  workflow_dispatch:
+
+concurrency:
+  group: publish-wiki
+  cancel-in-progress: false
+
+jobs:
+  publish-wiki:
+    permissions:
+      contents: read
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repository
+        uses: actions/checkout@v4
+
+      - name: Check that the English pages are current
+        run: python3 tools/build_wiki.py --check
+
+      - name: Publish the pages to the wiki
+        env:
+          GITEA_API_URL: ${{ gitea.api_url }}
+          SOURCE_REPOSITORY: ${{ gitea.repository }}
+          WIKI_CREDENTIALS: ${{ secrets.TOKEN_FOR_GITEA_WIKI }}
+        run: python3 tools/publish_wiki.py