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

.gitea/scoped_workflows/publish-wiki.yml

@@ -1,5 +1,10 @@
 name: Publish wiki
 
+# DEPRECATED (ADR-0009, MIL-008): repositories publish their wiki by calling
+# the reusable workflow publish-wiki-pages.yml. This scoped workflow stays
+# until every repository that has wiki/.publish has migrated; it is then
+# removed in a later major release.
+
 # Shared through scoped delivery: it runs in every repository of an
 # organization, but writes only where the repository has opted in with
 # wiki/.publish (ADR-0004). TOKEN_FOR_GITEA_WIKI is read from the organization