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/troubleshooting.md

@@ -26,11 +26,13 @@ never starts, or stops before the step) is a **runner** problem.
 | `TOKEN_FOR_GITEA is missing or empty.` | configuration | Add the secret |
 | `Could not determine the Gitea source repository.` | configuration | The run has no repository name; report as a defect |
 | `Expected at most one GitHub push mirror ... found N` | configuration | Keep one GitHub push mirror |
-| `TOKEN_FOR_GITEA_WIKI is missing or empty.` | configuration | Wiki publishing only: the repository opted in with `wiki/.publish` but its organization has no such secret; store it (see [credentials.md](credentials.md)) |
-| `wiki/.publish may only be empty or contain the word 'prune'.` | configuration | Wiki publishing only: leave the marker empty, or put only the word `prune` in it |
+| `TOKEN_FOR_GITEA_WIKI is missing or empty.` | configuration | Wiki publishing only: the repository calls the workflow but its organization has no such secret, or the call does not pass it; store it and pass it in `secrets:` (see [credentials.md](credentials.md)) |
+| `wiki/.publish may only be empty or contain the word 'prune'.` | configuration | Wiki publishing through the older scoped workflow only: leave the marker empty, or put only the word `prune` in it |
+| `The prune input must be 'true' or 'false'` | configuration | Wiki publishing only: set `prune` in the call to `"true"` or `"false"` |
+| `The wiki folder ... does not exist` | configuration | Wiki publishing only: set `wiki_dir` in the call to the folder with the pages (default `docs/wiki`) |
 | `... HTTP 403 ...` while publishing the wiki | permissions | Wiki publishing only: the token needs `write:repository` and write access to this repository |
 | `The wiki is not enabled on <repo>` | configuration | Wiki publishing only: enable the wiki in the repository settings |
-| `The wiki source directory ... has no Markdown pages` | configuration | Wiki publishing only: `wiki/` is empty or missing; nothing was changed |
+| `The wiki source directory ... has no Markdown pages` | configuration | Wiki publishing only: the folder has no Markdown pages; nothing was changed |
 | `Could not determine the GitHub owner and repository ...` | configuration | The mirror address must look like `https://github.com/<owner>/<repo>.git` |
 | `Gitea API request failed with HTTP 401` | credentials | Create a new Gitea token and update `TOKEN_FOR_GITEA` |
 | `GitHub API request failed with HTTP 401` | credentials | Create a new GitHub token and update `PAT_FOR_GITHUB`; the value must be the token only (no name, quotes or JSON) |
@@ -53,12 +55,14 @@ synced.` The run succeeds but nothing was synced. Check that the Gitea
 version supports the push mirror API and re-run.
 
 `No wiki/.publish marker in <owner>/<repo>; wiki publishing is not enabled for
-this repository.` The Publish wiki run succeeds and writes nothing. The
-repository has not opted in; add `wiki/.publish` if it should publish.
+this repository.` The older scoped Publish wiki run succeeds and writes
+nothing. The repository has not opted in; call the reusable workflow
+**Publish wiki pages** (see [onboarding.md](onboarding.md)) if it should publish.
 
-`<n> page(s) are in the wiki but not in wiki/ and were kept; put the word
-'prune' in wiki/.publish to delete them.` The run succeeds. Pages that exist
-only in the wiki are left alone unless the marker contains `prune`.
+`<n> page(s) are in the wiki but not in <folder>/ and were kept; set the prune
+input to true to delete them.` The run succeeds. Pages that exist only in the
+wiki are left alone unless the call sets `prune: "true"` (with the older scoped
+workflow: the word `prune` in `wiki/.publish`).
 
 ## Runner problems