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/Troubleshooting.md

@@ -32,7 +32,8 @@ 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 |
-| `No token for the wiki: set TOKEN_FOR_GITEA_WIKI, or TOKEN_FOR_GITEA.` | configuration | Wiki publishing only: add one of the secrets (see [credentials.md](Credentials)) |
+| `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)) |
+| `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 |
 | `... 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 |
@@ -57,6 +58,14 @@ settings.
 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.
+
+`<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`.
+
 ## Runner problems
 
 See the [Runners](Onboarding#runners) section. In short: if the run stays