Remove the scoped publish-wiki workflow and the marker path (2.0.0)

- .gitea/scoped_workflows/publish-wiki.yml is deleted
- tools/publish_wiki.py requires WIKI_PRUNE, reads no marker, and defaults
  WIKI_DIR to docs/wiki; its tests follow
- README, onboarding, troubleshooting and upgrading guides, and the wiki pages
  in English, Danish and Bahasa Malaysia no longer describe the scoped workflow
- changelog entry 2.0.0 with the Breaking group

Task: MIL-009#6
Task: MIL-009#8

guides/onboarding.md

@@ -79,10 +79,6 @@ The wiki has English and the product owner's language as a minimum.
 5. Merge the change to `main`. The log must end with `Published <n> page(s) to
    the wiki of <owner>/<repo>`.
 
-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
 
 | Item | Value |