Publish the wiki through a reusable workflow; this wiki moves to docs/wiki

- publish-wiki-pages.yml: reusable workflow with wiki_dir, prune and
  tools-ref inputs and the declared secret TOKEN_FOR_GITEA_WIKI
- tools/publish_wiki.py: WIKI_PRUNE selects explicit mode without the marker;
  a missing folder is a configuration error
- publish-own-wiki.yml: this repository calls the reusable workflow
- wiki/ moves to docs/wiki/; build_wiki.py and its tests follow
- the scoped publish-wiki.yml is marked deprecated, not removed
- guides, README and the Danish and Bahasa Malaysia wiki pages describe the
  caller, docs/wiki/ and the new error messages

Task: MIL-008#2
Task: MIL-008#4
Task: MIL-008#5
Task: MIL-008#6

guides/onboarding.md

@@ -45,25 +45,43 @@ Delivery is automatic, so a repository needs no copy of the workflow. Secrets ar
 
 ## 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.
+The reusable workflow **Publish wiki pages** copies the Markdown pages in
+`docs/wiki/` to the repository wiki when a change to `docs/wiki/` is merged to
+`main`. The repository calls it, so nothing is written where no call exists.
+The wiki has English and the product owner's language as a minimum.
 
 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. A hyphen in the name is a space in the page name: `Home-da.md`
-   is the page "Home da", and a link to it is `[Dansk](Home-da)`.
-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.
+2. Add Markdown pages to `docs/wiki/`. The file name is the page name: `Home.md`
+   is the home page. A hyphen in the name is a space in the page name:
+   `Home-da.md` is the page "Home da", and a link to it is `[Dansk](Home-da)`.
+3. Add a workflow, for example `.gitea/workflows/publish-wiki.yml`, that calls
+   the reusable workflow:
+
+   ```yaml
+   on:
+     push:
+       branches: [ main ]
+       paths: [ "docs/wiki/**" ]
+   jobs:
+     wiki:
+       uses: TirSystem/github-action/.gitea/workflows/publish-wiki-pages.yml@v1
+       with:
+         prune: "true"
+       secrets:
+         TOKEN_FOR_GITEA_WIKI: ${{ secrets.TOKEN_FOR_GITEA_WIKI }}
+   ```
+
+   `wiki_dir` defaults to `docs/wiki`. `prune: "true"` also deletes wiki pages
+   that are not in the folder, so the wiki always equals the folder; the
+   default `false` keeps them.
 4. Store `TOKEN_FOR_GITEA_WIKI` as a secret of the repository's organization,
    using the scope in [credentials.md](credentials.md).
 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.
+A repository that still has the marker `wiki/.publish` is published by the
+older scoped workflow, which is removed once every repository has a call.
+Move the pages to `docs/wiki/` and add the call before that.
 
 ## Runners