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

README.md

@@ -25,6 +25,7 @@ It currently contains the Gitea workflow [`sync-github-metadata.yml`](.gitea/sco
 - Repository secrets:
   - `TOKEN_FOR_GITEA`: Gitea API token (plain string, or JSON with `GITEA_TOKEN`)
   - `CREDENTIALS_FOR_GITHUB`: JSON object containing `GITHUB_PAT`, a GitHub personal access token allowed to edit the mirror repository
+  - `TOKEN_FOR_GITEA_WIKI` (only in this repository, for the **Publish wiki** workflow): Gitea token that can write the repository wiki
 
 ## 🛠️ Setup