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

tests/test_workflows.py

@@ -29,7 +29,8 @@ class TestEachWorkflow:
     def test_every_job_declares_permissions_and_runner(self, path):
         document = yaml.safe_load(path.read_text(encoding="utf-8"))
         for name, job in document["jobs"].items():
-            assert "runs-on" in job, f"job {name} has no runs-on"
+            if "uses" not in job:  # a called workflow has its own runner
+                assert "runs-on" in job, f"job {name} has no runs-on"
             assert "permissions" in job, f"job {name} has no explicit permissions"