chore: remove the remaining VuePress artifacts

Follows the devDependency removal. The build has been mdPress-only since
1.7.0 and nothing referenced these:

- .vuepress/ (config.js + .gitignore). The mirror it configured is still
  live, but no workflow in this repo builds or deploys it, so this was a
  recipe nobody ran. Local .vuepress/dist/ (400 files of Feb-2026 build
  output, never tracked) was removed from the working tree too — with its
  .gitignore gone it would otherwise have shown up as untracked noise.
- docker-compose.yml: the vuepress-offline service, plus the now-unused
  &mdpress-offline anchor that existed only to feed it.
- README: the 'docker run ... :vuepress' one-liner. That tag was last
  pushed 2022-06-25, so readers following it got a four-year-old snapshot
  of the book. 'mdpress serve' immediately above it still works.
- CODEOWNERS, .zhlintignore and check_project_rules.py entries for the
  deleted directory, and the check-link allowlist entry for the mirror
  (no content links to it).

The CHANGELOG entry recording the original CI removal is left alone — it
is history.

Verified: npm test passes (3 + 35), check_project_rules passes 202 files,
docker-compose.yml still parses with no dangling YAML aliases.

docker-compose.yml

@@ -15,12 +15,7 @@ services:
 
   # docker run -it --rm -p 4000:80 dockerpracticesig/docker_practice
   mdpress-offline:
-    &mdpress-offline
     # this image build by GitHub Action
     image: dockerpracticesig/docker_practice:mdpress
     ports:
       - 4000:80
-
-  vuepress-offline:
-    << : *mdpress-offline
-    image: dockerpracticesig/docker_practice:vuepress