Document how a repository opts in to wiki publishing

Describe the wiki/.publish marker and the prune word, the per-organisation
TOKEN_FOR_GITEA_WIKI without fallback, and the new messages in the
onboarding, credentials and troubleshooting guides and the README. Add the
capability row and regenerate the five English wiki pages with their Danish
and Bahasa Malaysia translations.

Task: MIL-006#5

wiki/Onboarding.md

@@ -49,6 +49,27 @@ Delivery is automatic, so a repository needs no copy of the workflow. Secrets ar
    and topics.
 7. If it fails, use [troubleshooting.md](Troubleshooting).
 
+## Publish a wiki (optional)
+
+The **Publish wiki** workflow copies the Markdown pages in `wiki/` to the
+repository wiki when a change to `wiki/` is merged to `main`. It is delivered
+like the sync workflow, but it writes only where a repository opts in.
+
+1. Enable the wiki in the repository settings.
+2. Add Markdown pages to `wiki/`. The file name is the page name: `Home.md` is
+   the home page.
+3. Add the marker `wiki/.publish`. Leave it empty to create and update pages
+   only. Put the single word `prune` in it to also delete wiki pages that are
+   not in `wiki/`, so the wiki always equals the folder.
+4. Store `TOKEN_FOR_GITEA_WIKI` as a secret of the repository's organization,
+   using the scope in [credentials.md](Credentials).
+5. Merge the change to `main`. The log must end with `Published <n> page(s) to
+   the wiki of <owner>/<repo>`.
+
+A repository without `wiki/.publish` is skipped with the line `No
+wiki/.publish marker in <owner>/<repo>`: nothing is written and no secret is
+needed.
+
 ## Runners
 
 | Item | Value |